Edit: Don't miss /u/drylaw's piece below - or above, depending on your sort - as while I focus more on Texas and the US, his provides an excellent complement looking more at Mexico!

Distance. The vast majority of flights for freedom were from the upper South, where the enslaved persons were imprisoned within a a not unreasonable distance to the North. If you consider some of the most famous people who were born in bondage, they mostly originated there. Harriet Tubman was owned by a Maryland enslaver, as was Frederick Douglass. Slipping North was the only real viable option for them, while traversing the entire South would have been entirely out of the question except in the most exceptional circumstances, and speaks to the general plight of those persons enslaved in the Deep South who simply had no opportunity. The famed flight of Ellen and William Craft illustrates just how tough it could be, their successful escape from a Georgia plantation being possible only because Ellen, a light-skinned woman who would have been a "quadroon" in the racial parlance of the time, was able to successful pass as a white man and her husband as her slave for the train journey northward, essentially traveling in the open, the one way to beat the odds of traversing such a large swathe of country populated by a hostile population and slave patrols looking for any slave without a pass.

So in short, you have enslaved persons from the upper South with no logical alternative, and those in the Deep South often with little chance of escape no matter what - many who did, in fact, wouldn't head North OR to Mexico, but rather would settle-down in the large, nearly impenetrable swamps that characterized much of the region, carving out a free existence as a small enclave within the slave south. But for those in Texas? Well, it is the Upper South in reverse, Mexico of course was where they were headed. As you note, slavery was illegal there - the desire to ensure its security being a key reason Texas broke away - and this made it an obvious destination for any enslaved person attempting to find freedom.

It was enough of a problem for the laws of Texas to reflect the threat posed by the border to the institution of slavery. Texas has several laws in place that dealt with the apprehension of those seeking their freedom, but included extra enticement for those close to attaining it, as an 1844 state law entitled anyone who caught an escaped slaves west of the San Antonio River to "a fifty-dollar reward for each plus two dollars for every thirty miles traveled to return them to the rightful owner. This was bolstered in 1858 law, allowing the person who captured a slave escaping to Mexico to be paid 1/3 of their value by the Travis County Sheriff, who would in turn be repaid the amount by the owner, or by resale of the person. Although "An Act to Encourage Reclamation of Slaves Escaping Beyond the Limits of the Slave Territories of the United States" didn't explicitly do so as it couldn't openly challenge Mexican sovereignty, the law was well understood to be offering enough financial encouragement to potential slave-catchers that they would be willing to risk going south of the border for their quarry.

The latter law especially was a clear reaction to the opinions of Texans as regarded their neighbors to the south, as the Mexican government of course did nothing to return escapees, and Texans felt that many Mexicans were sympathetic and assisted those who fled, which was bad enough, but for those who remained in Texas such things could stand to “stir up among our servants a spirit of insubordination." The inducement of escape also was feared to be fuel to the ever present terror of servile insurrection. An 1856 series of newspaper articles claimed to have uncovered a plot in Colorado County by a group of slaves to murder most of the whites, and flee south to Mexico, dragging along the young white women as captives for obvious but only insinuated purposes. Although there likely was some group of slaves planning to flee their prison camp, claims were made of intricate organization that would make a Mason proud, and there is little evidence, beyond the writers' imaginations, that a mass group of slaves was:

organized into companies of various sizes, had adopted secret signs and pass-words, sworn never to divulge the plot under the penalty of death, and had elected captains and subordinate officers to command the respective companies.

Of course, truth is beyond the point though, and what mattered was the impact on the white population. Conventions, such as that held in Gonzales County in 1854, were held to discuss the issue, the organizers there declaring:

The escape of our slaves into Mexico by the help of Mexicans and otherwise has become a matter of magnitude, and of sufficient importance to demand some decided action on the part of the people of Western Texas.

In total, we can't put a precise number on how many chose the path of Freedom via Mexico, but certainly it numbered in the thousands. Even in Texas though, in the northern part of the state Mexico wasn't a guarantee, and some chose to make their bid by aiming for Indian Territory (modern Oklahoma) or further north to find sympathetic abolitionists in Kansas, but what records we do have would show them to be the minority. Mexico was the best option and the choice for most Texas escapees. But to tie back into your question, it was an option for few others. It was a tough enough journey through the Texas wilderness, such as for the group of 25 enslaved persons from Bastrop who requisitioned horses in 1845 and made their dash, but the odds of even reaching Texas, let alone Mexico, from Mississippi or Alabama, let alone the upper South, was far too slim, and whatever the appeal it might offer for freedom simply wouldn't outweigh the practical realities of affecting escape in the first place.

Barr, Alwyn. The African Texans, Texas A&M University Press, 2004.

Blackett, R.J.M. Making Freedom: The Underground Railroad and the Politics of Slavery. University of North Carolina Press, 2013.

Gara, Larry. The Liberty Line: The Legend of the Underground Railroad. University Press of Kentucky, 1961

Gillmer, Jason A. Slavery and Freedom in Texas: Stories from the Courtroom, 1821–1871, University of Georgia Press, ATHENS, 2017, pp. 181–224.

Laws of Slavery in Texas: Historical Documents and Essays, edited by Randolph B. Campbell, William S. Pugsley, and Marilyn P. Duncan. University of Texas Press, 2010.

Follow up question:

Did many slaves actually flee to Mexico? Is there any writings on how they adapted and integrated into the culutre? And furthermore, are there now people living in Mexico who are mixed Mexican and African due to these slaves from the Civil War?

u/Georgy_K_Zhukov has already discussed in detail why the majority of escaped slaves went North - my focus here is more on why and how Mexico became an important destination for slaves from Texas.

​ Slaves did flee from what is now the U.S. over the Mexican border, before and after the annexation of Texas in 1845. As you mention newly-independent Mexico had abolished slavery in 1829, although with exceptions. The number of slaves crossing the border increased after 1845 and even continued after the emancipation in the US, with Mexican attitutes towards slavery and race seen as an opportunity for more freedom by African Americans. Estimates place around 4.000 slaves migrated to Mexico by 1855.

Sean Kelley has written an interesting article on the topic ("Mexico In His Head": Slavery and the Texas-Mexico Borderm 1810-1860) which I'm drawing from here, where he describes four periods in how the Mexican border became connected to non-slavery:

1) Until ca. 1820, whith an undetermined border between US and New Spain (and then Mexico), and slavery legal in both regions so that only few slaves fled to Texas to escape their masters.

2) 1829-1845, the beginnings of plantations slavery, but only dimly connected to Mexico and freedom there.

3) Ca. 1829-1845, with tensions escalating between Anglo Texans and the Mexican gov't over various issues including slavery, leading to the annexation of Texas as a slave state. 1000s of slaves fled across the Río Grande in hopes of finding freedom.

4) Following emancipation, Mexico continued to serve as an image that "the racial hierarchies of the postwar South were by no means natural, inevitable, or just" [Kelley, 710]. Let's look at these steps in some more detail.

In colonial Mexico aka New Spain slavery was an important institution from the beginning, as overall in the Americas. The demographic crisis of the native population meant an increase of African slaves brought to New Spain, with Cortés initating the first transport in the early 1520s. Slaves formed a crucial part of the colonial economy, although many managed to escape and formed settlements throughout central America, often living together with native groups, sometimes throughout colonial times (their descendants form an important part of today's Afro-Mexicans).

By the 18th century population growth among indigenous people together with an increasing mixed population made slavery less important, so that by some estimates at the time of independence in 1821 only about 3.000 were left in New Spain, with a very small number of them in Coahuila-Texas.

The Mexican War of Independence meant a re-settling of the northern border, with a new border set up by Spain in the US already in 1819. It also impacted on the issue of slavery there. The early main leaders of independence, Hidalgo and Morelos had both in different ways called for slavery's abolishment. During the short presidency Vicente Guerrero slavery was abolished in 1829. Guerrero himself was the first North American president of mixed descent, including African heritage.

While the Hidalgo movement had lost momentum early on, antislavery rhetoric continued throughout the 1820s, with no Mexican slaveholders to object. However, treaties passed during the late 1820s were quite ambiguous, e.g. with Texas being exempt from the abolishment of slavery.

The abolishment in Mexico led to different reactions among northern slaves, with Coahuila-Texas now a slaveholding society: some went looking for freedom further South in Coahuila or Tamaulipas. Others looked to the burgeoning slaveholding regions for escape, which were still more sparsely settled at that time. But a few years later the only choice left was to cross the Río Grande. ​​ ​Mexican equivocal attituted towards slavery led to Mexico taking a special place in slave's imaginations. Slaves would routinely reinterpret pronouncements made by the Mexican government, forming Mexico into a symbol of non-slavery.​ In addition the Mexican pronouncements further led to Texan slaveowners viewing Mexico as a poteintial threat.

​With tensions between Texas and Mexico boiling over in the 1830s, increasing numbers of slaves fled south. ​While some of them were sent further south, others had less luck and were sent back to the north - always depending on the Mexican general in charge. Nonetheless, "as with the issue of legality, what mattered was not the actual policy, but the significance slaves attached to Mexican equivocation. To them, the Mexican Army was an army of liberation." [Kelley, 716]

With the anexation of Texas, slaves continued to escape south, probably numbering in the thousands. While both state and private individuals undertook expeditions to bring fugitives back, Mexican opposition tended to squash such attempts. But there was also resistance by the slaves themselves, taking different forms: a slave revolt took place in 1835 at the Brazos river, brokend down by the militia. And a supposed plot by 200 local slaves to kill their masters was discovered in 1856 in central Texas, leading to the hanging of three "leaders". Since this fits well with fears of supposed slave rebellions in the Americas it's hard to say how much of this plot was real, but it again shows the linking of Mexico with antislavery discourses.

Even after 1845 then, Mexico continued to be viewed by slaves as a republic built on a more inclusive model of citizenship - a view that can cetrainly be debated, but certainly speaks to better perspectives at least for people of African descent there compared to the Jim Crow South. Regarding the continuing Río Grande crossings, Roseann Bacha-Garza observes that often families and towns would help escaped slaves, providing them with resources, money and food. Due to the region's complex history there seems to have been more empathy for escaped slaves than in other southern regions. However military forts were also present along the river, remnants of the Mexican-American War. Officials at these forts were instructed to capture and return any slaves to their owners, but the structures were far from each other, making a clear border patrol difficult to maintain.

Kelley notes that even into the late 18th and early 19th centuries, communities of slaves continued to appropriate the language of freedom across the Americas, drawing on a variety of subversive texts:

Among these texts were political boundaries, not only the Mexican border, but the Haitian coast, the Mason-Dixon Line, the Ohio River, and eventually the picket lines of the Union army. Slaves were among the world's most persistent border crossers. [Kelley, 719]

Edit: added a paragraph

Hi folks,

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language barriers, social barriers, cultural barriers, there were already kin in the north etc.. I wouldnt go to mexico simply because of the language barriers. what would i do in mexico when i couldnt even speak the language? starve?