The answer to that goes back to the early days of the recording industry. You're right, country, blues, gospel and southern gospel artists all borrow relentlessly from each other, and that tradition goes back to the 1800's before the genres got clearly separated out (I can't speak to mariachi or norteno as I haven't studied those). As I understand it, in the early 1900's as the recording industry was coming online, "race" records and "hillbilly" records were created as niche markets - race records selling to lower class African Americans, hillbilly records selling to lower class white Americans. The musics sounded very similar at that point, but it was the marketing of the artists that separated them out by race. Now whites were more than happy to listen to black music, and vice versa, but it was how they were marketed that created the initial distinction between the genres, which became country and blues, respectively, and once distinguished they became statements of identity. Their religious counterparts, gospel and southern gospel, have a slightly different history, as they both emerged from hymns but southern gospel also was influenced by rural singing conventions. Each of those genres borrowed from each other, but were more clearly separated even from the beginning because Sunday morning is the most segregated hour of the week in the U.S. - much less opportunity for artists to share music, although once the Pentecostal movement got going in the early 1900's, that allowed for a lot more crossover since it started out inter-racial.
Why it's remained that way is simply that music is a very powerful means of creating a sense of personal and group identity. Music, of all kinds, acts physically (resonance of sounds in the body), emotionally (connection of sounds to emotional states), and textually (in the lyrics) to reinforce particular social narratives and ways of being in the world. Once country music was successfully labeled white by the marketing arm of the recording industry, that took on a life of it's own thanks to the racial dynamics of American culture. It becomes a self perpetuating cycle where whiteness becomes a significant feature of country music, and vice versa, as a shortcut for distinguishing between and reinforcing the social and racial class structures. Food, clothing, and other cultural items work much the same way.
Some Sources:
To give a short answer: while these early forms were similar, segregation of performers both by music producers and American society in general, contributed to a divergence in genres, though blues continues to be an incredibly important influence on country
Early on in the history of American music, you're correct that what evolved into country music borrowed heavily from especially blues and gospel, as well as other forms like tejano and cajun music. It is also important to note that these genres also borrowed from each other. Similar themes and songs (trains, the legend of John Hardy, etc.) appear all across what could be termed folk music in the 20's and 30's.
Potentially the biggest industry-produced effect that pushed along these schisms was the distinction between white and "colored" artists early on in recording history. Music by black artists and marketed for a black audience was "race music," while music by white artists and marketed for a white audience was "hillbilly music." Other music forms you mentioned that were popular within Hispanic and Latino communities were recorded but, since the bigger artists in those genres sang in Spanish, those styles had little popular appeal until the Latino population grew later in the century. My knowledge of early Latino music is a incomplete so if I am wrong, please correct me.
The Blues-Country schism was also reinforced by segregation and the institutionalization of both genres. Early in music recording history, it would've been unusual to find an artist of either race who sang only country or only blues. But the broad local appeal of the sounds led to commercial success, and of course there were forces which wanted to capitalize on that success. The WSM Barn Dance started in 1925, but its name soon changed. In 1927, while introducing African American harmonica player DeFord Bailey, announcer George Hay called the program the "Grand Ole Opry" for the first time, and it would eventually become the dominant stage in country music and end goal for aspiring country artists. I have no knowledge of the Ryman Auditorium, the Opry's home, being segregated. However, it is likely, though hard to determine, that small-time black country artists had reduced appeal due to the Jim Crow laws of the states they performed in, which contributed to the appearance of country as being white.
The two genres also developed differently and diverged. Blues gave birth to jazz. A jazz-infused country styled called Western Swing was born in the 40's. The Grand Ole Opry became able to determine what styles would get national exposure. In a way, what the Opry said was country was what became country. By the mid 40's to mid 50's, this was Honky Tonk. This was a sound that was blues-influenced but not similar to blues the way old hillbilly music was. Still, in the Honky Tonk era, Blues had a tremendous impact on country music. Hank Williams, a white country music star (one of the legends of country music), learned guitar from a blues musician. But the blues forms that impacted country no longer had appeal to the urban African American in post-Great Migration Chicago and it would take the electrification of the blues to reintroduce it as a popular form. Later, the country music establishment purposefully changed styles to a mass-appeal Nashville Sound which kept little from the Honky Tonk style immediately preceding and had little visible blues influence, so even the older blues players had little interest in mainstream country anymore.
There have been black artists who have been influential in country music, but it wasn't until the 60's, with Ray Charles releasing an album of traditional country songs and Charley Pride becoming the first black performer on the Grand Ole Opry since DeFord Bailey's firing in 1941. However, the introduction of Outlaw Country, whose performers sometimes released songs glorifying the South, which was sometimes tied to the Civil War, understandably turned a lot of Black listeners away and towards other sounds like funk, disco, and what would eventually become hip-hop.
Black country artists exist today, but the location of country's center in the South, the harsh Jim Crow laws in much of country's market, the controversial inclusion of the Confederacy in some styles of country, the early industry-driven distinction between blues, country, and other folk music, and the shifting interest of black audiences away from both country and blues, led to country being a white-dominated genre.
PBS I believe recently did a nice miniseries called American Epic that explores a few different artists of the 20's and 30's across a broad range, including country, blues, tejano, and hawaiian.
This is also like my second answer on here so let me know if I can do better. It's nice to see American folk musicology get covered.
EDIT: Thought I might add how fitting this question is for the theme, as I generally consider the 1940s to be the time where blues and country significantly diverge stylistically.
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Hi!
I'm actually doing a lot of research about Nashville, Austin, country music, and authenticity right now so I'm in the thick of this question. I think other posters have covered an in-depth history of the genre and the development of the music itself, but you might find some of this interesting.
Country music, as a genre, was marketed heavily to white Southerners in the first half of the twentieth century as a way to connect to rural roots. It served as a form of cultural validation for self-conscious Southerners in a time where you see a lot of Southern pride movements occurring. The South was attempting to demonstrate to the rest of the country that the region was competitive in education, technology, and industry. There was a distinct urban/rural cultural divide; members of the "cultural elite" of large Southern cities wanted to distance themselves from the rural images traditionally associated with the South. In Nashville, the National Life Insurance Company started a high-powered radio station called WSM in 1925. The station played mostly opera and classical music, but began playing a "barn dance" style program where "old-timey" or "hillbilly" musicians would play traditional music and act as hillbilly characters on air. One day, a classical music program out of Chicago was playing on the air. The announcer quipped that there was "no place in the classics for realism." WSM's barn dance program ran directly after, and announcer George Hay gave the Grand Ole Opry its name:
“…for the last hour, we have been listening to music taken largely from grand opera and the classics, and heard Dr. Damrosch tell us there in so no place for realism in that kind of music. In respectful contrast to Dr. Damrosch’s presentation, for the next three hours we are going to present nothing but realism.”
and, later, satirizing the high-brow attitude of the opera program:
"From now on we will present the Grand Ole Opry."
Thus begins the official association with country music and authenticity. While Hay would often have African American artists on his show (the previous comment was actually made directly after a performance by an African American harmonica player), it's important to understand that this program spoke to the identity of white southerners at a time when you see a lot of "old south" longing occurring (Gone With the Wind, confederate monuments, even the resurgence of the KKK). White southerners, particularly those in the working class, were tired of feeling like their culture was wrong or undignified. This isn't my area of expertise, but I'd argue that the use of African American sounds in country music was legitimized because white working-class southerners felt like this music was "authentic" because of the working-class roots of African Americans.
The history of the recording industry in Nashville is long and messy, but one of the most interesting things I read today applies here. Historian Jeremy Hill suggests that in the midst of the civil rights movement, the country music industry marketed music to "ordinary white folks" without directly claiming any racial preference. He quotes Tandy Rice, who was later president of the Country Music Association. Rice was asked in 1967 about the appeal of country music and how it contrasted with leftist policies:
"Right now, country music is stable, like the great backbone of the country. The lyrics are simple, and sincere, not about civil rights and such. These folks don't go for the Bob Dylan, Joan Baez kind of thing. The lyrics are about what concerns everyday folks."
Now, country music has changed drastically over the years- I'm particularly interested in the countercultural "Cosmic Cowboy" movement that occurred in Austin in the 1970s- but the core of the genre has always been a white search for identity. In short, country music wasn't just marketed as "southern" music. It was marketed as white southern music.
Sources:
Jeremy Hill, Country Comes to Town: The Music Industry and the Transformation of Nashville. This book in particular covers the relationship between race, country music, and Nashville.
Craig Havighurst, Air Castle of the South: WSM and the Making of Music City, USA
Joli Jensen, The Nashville Sound: Authenticity, Commercialization, and Country Music
And if you're interested in the Cosmic Cowboy movement in Austin, I really like Jason Mellard's Progressive Country.