Well, it goes back to the same old argument, the simple fact is, Testament and Slayer (especially) where there first. Now of course that doesn't change the fact that people might like post 90's stuff like Pantera more, but it will always go back to the original sound being more unique when it came out, less saturated, and sounding just a fuckload more genuine.
By the way, librariusmetallicus mentions the following:
While the music got heavier, the lyrics got worse, relying too heavily on cussing for shock value. Of course, this only increased their popularity. At the peak of their career, they were one of the most popular metal bands in the world
Gotta ask, did anyone outside of the US even know who Pantera was, even as late as 96?