Heinrich Schenker was born in 1868 and died in 1935. And unfortunately, it's probably something of a blessing that he died when he did: as a Jew living in Austria in the 1930s, he only just missed the annexation of Austria (1938) and the subsequent horrors that that brought. Schenker's wife, for instance, died in Theresienstadt (a "hybrid concentration camp and ghetto") in 1942.
I say this to lay the context of how Schenkerian analysis first came to the United States: many of Schenker's students, perhaps chief among them Felix Salzer, emigrated to the United States as a means of escaping the war. Salzer then went on to teach at the Mannes College of Music and Queens College (part of the City University of New York). Other students that emigrated to the US include Oswald Jonas and Ernst Oster (who taught at Mannes as well as at the New England Conservatory).
Schenker's theories developed over the course of a few decades, but right as his theories were reaching maturity, his biggest proponents either left Austro-Germany for the US or, unfortunately, died in Europe.
As such, the Schenkerian tradition began to flourish in the United States while Austria and Germany continued using prior methods of musical understanding: thoroughbass, Riemannian function, and so on. And believe it or not, we still see this divide today.
This is also why we see so many Schenkerian-influenced schools (CUNY and Eastman come to mind) in New York state and in the northeastern United States in general: this is where these musicians lived when they came to the United States, so naturally this is where the theories now have the strongest footholds.
As for the Journal of Schenkerian Studies being located in Texas, this is just because that school has a particularly active Schenkerian department; many of the scholars there are Schenkerians, so creating a journal there was a natural outgrowth of that.
Schenkerian theory isn't exclusive to the United States, but it's certainly more common there than in other countries (with Britain being a close second). The fact is that American academic music theory is quite different from, say, academic music theory studied in Germany. But as our world becomes closer and closer, we're starting to see Schenkerian theory studied elsewhere, as well.
And I should say that Schenkerian theory isn't just something for "academic" music theorists: Schenkerian concepts are in undergraduate textbooks by Aldwell/Schachter, Laitz, Clendinning/Marvin, Gauldin, and so on. The Kostka/Payne textbook is probably the book least influenced by Schenkerian theory. (Note that these are all textbooks written by Americans, but only because those are the texts that I'm familiar with.)
Lastly: I just recently watched the Adam Neely video, and it was very well done. But there were a few moments where I felt he misrepresented the world of academic music theory, and someone not of that world that only watched the video could walk away with a viewpoint of music theory very different from reality. Again, his video was great, and everything he said about Ewell's white racial frame and the white supremacist background of music theory was spot-on. But there were other moments where, either due to simplification or due to his own misunderstandings, he missed a fair amount of nuance in the world of academic music theory that makes it look much less healthy than it really is. I don't have time now to re-watch the video and provide any commentary, but I may do so in the future.
