No need to feel sorry for the norteño - after all, all music is a product of adaptation and mixing of older forms and styles. Your question made me curious so I looked into it - I'll mostly draw on Luis Díaz Santana Garza's detailed "Historia de la música Norteña Mexicana".

Two things to keep in mind: First, the border region between the Southern US and Northern Mexico continued to be a region of cultural exchange, including arts and music. Second, while norteña music goes back to central European immigration as we will see, these European musical influences were also adapted to Mexican traditions and transformed over time.

European musical migration

The large-scale migration of German, Polish and Czech people in the 19th c. especiallz to Texas and Nuevo Leon would have a major impact on the musical development. As an example, a large migration of Germans to Southern Texas took place in the 1830s and 1840s. They would make up the largest ethnic group in Texasof European descent, and by 1850 Germans made up more than 5 percent of the entire population of Texas (this region becoming known as the "German Belt").

Importantly for us, this extended to cultural influence. We can see this in the construction of a large number of Czech cultural centres in Texas between the late 19th c and the 1980s, offering dances and concerts. German dance saloons were also opened in the whole state, open to various musical genres from Mexico and the US.

Before looking at the polka, it's interesting to note the influence of European marching bands in Mexico in the 1860s - due to the short-lived Hapsburg reign of Maximilian I. While these bands and orchestras played rather classical music and military marches, they can nonetheless be seen as bringing a stronger European influence and "professionalization" (for lack of a better word) to Mexican musicians. Such orchestras were also stationed in northern regions like Monterrey, where they were supposed to win favor for the often despised government of Maximilan. Another features of such groups was the gradual adoption of local instruments, used together with more "classical" ones, which we will also see with the norteño.

Polka in Mexico

One of the most popular dances in these saloons since the 19th c. has been the Polka. Its exact origins are unclear (basically everyone from the Polish to the Czech now claiming its invention), but Santana Garza highlights that the major influence in the Mexico/US comes from German and Czech polka, which relied heavily on the accordeon. In contrast, Polish polka featured/features the violin more heavily.

Towards th mid 19th century, Iberian dances lost polularity rapidly, surely in connection with the wars of independence and the anti-Spanish sentiments they brought. At the same time, the polka and other European musical styles spread throughout the whole American continent. Another example of this would be Argentinia, where polka can be traced to at least 1845, and where the accordion would of course also play in important role in genres like the tango. Santana Garza mentions how from the late 1840s there kicked off a veritable polka "craze" (furor) in Mexico, with the word being used for a certain type of male pants, and at one moment polkos even being applied to all things "strange and extravagant" (crazy how word meanings have changed).

Around this time, with the global popularity of polka, we see the polca norteña coming up, comprised of a conjunto or band. According to Manuel Peña, the música ranchera spread by these conjuntos is nothing else but a polka or a waltz, with a sung melody inserted. However by the 1930s at least in the cities this popularity of polkas was fading. Once more, this had a lot to do with political changes: At least for some citizens of post-revoultionary Mexico, these dances were tied directly to the hated ancien régime. These changes also led to a stronger modification of polka-derived styles, tied especially to two instruments: the accordion and the bajo sexto ("sixth bass").

Instruments: Accordion and bajo sexto

I won't go into too many details here on these instruments, but rather focus on their relation to the norteña. By the 1940s, typical istruments of Norhtern Mexican music like the clarinet and the tambora de rancho started being replaced by the accordion and bajo sexto. Actually, first notice of the accordion in Mexico comes from the US American invsasion in Monterrey in 1847. I have to put some nortena music in, so here goes Acordeón by Lalo Mora, praising the merits of this instrument.

While the accordion is clearly a European creation, the bajo sexto is rather described as a hybrid or "mestizo" isntrument. It can be used to play bass lines, but also accompany with chords. Its origins are not entirely clear, but its usually traced to the 1920s, with production starting in Guanajuato and Mochoacán in Mexico. It bears resemblance to both European style 12-string guitars and to the Spanish guitarra barroca. This is a later photo from the 1960s, but we can still see pretty well the different instruments played by the conjunto norteno "Los Regionales del Bravo".

With the rising popularity of these instruments, the originally Mexican bajo sexto started to be produced in America as well due to high demand. Here we can see the start of the transformation of more European-style polkas into the norteno, with the addition these comparatively "new" instruments (including others like the tambora, and in later times upright or electric bass). The major diffusion of norteño initially happened in Southern Texas and near the border region, due to three factors: The technologies of and the radio, the migration of musicians from northeastern Mexico fleeing the post-revolutionary misery, and the identification of the mexican Americans with the music, who adopted it as a new form for ethnic and cultural identity.

Summing up

One major influence on the development of the norteño was, as you mention, the polka. Of special importance was German and Czech migration to Texas and Northern Mexico from the mid-19th century onwards. To the increasingly big role of the European accordion in this music we have to add the role of more native instruments like the Mexican hybrid bajo sexto - both instruments forming the base of traditional conjuntos norteños. What is more, back in the 1860s the influence of European marching bands under emperor Maximilian I. added to the importance accorded to professional musicians and European instrumentation alike.

This development of a new genre falls together with first "substantial and permanent" migration towards the North, following the Mexican Revolution. In Texas arrived mainlz habitants of Coahuila, Nuevo Le¡on and Tamaulipas. Only in the first 3 decades of the 20th c. circa 1,5 million Mexicans crossed the border, settling primarily in the American southeast. Together with central European migration to these parts a good century earlier, here we can see another main reason for why norteño would proliferate first in Texas with the conjunto tejano, from there spreading further into other American and Mexican regions.

Edit: Just had to add a narcocorrido here as the bastard offspring of the norteno (although it's outside the 25 years rule).