Look at the first few years compared to today, and tell me that you don't see an obvious shift in style. For every REM, Spoon, and Shins that were on the early bills, there were 10 roots acts. Today, it's the mainstream acts that predominate, and the roots acts are nearly an afterthough.
Besides, Spoon *IS* a Texas homegrown band, and REM were a southern band with jangly guitars. Both were closer in spirit to the tv show than LCD Soundsystem and Muse, to choose a couple of examples.
http://en.wikipedia.org/wiki/Austin_City_Limits_Music_FestivalMy point was the the television show for many years, and in turn the festival, was essentially a roots/Americana showcase. Now it is much more diverse. It's much more of a "little bit of something for everyone" show/festival and much less a niche show/festival. You're lauding that fact, I'm lamenting it.
While the *show* is like 30 years old and originally focused on Texas (homegrown) music, the *festival* is only 8 years old. REM and Spoon played the second year of the festival, and it seems like while association with the show set a tone for the first year or so, concert promoters will make money with whatever band will draw the crowds...there's no long, well-established tradition at the ACL *Festival*