for me it's the fact that you can get something like Pappy 12 for the same or less money than the Garrison Bros.
I'll admit I haven't had the chance to try it, not living in Texas, but from pretty much everything I've heard it's not worth anywhere near the price they charge. That fact, I think, makes many resent the fact that you're basically getting a $25-$30 bottle of bourbon, and paying another $50 because it's marketed so prominently as "Texas" Bourbon, essentially exploiting the state loyalty of Texans.
Just my observations after having followed many Garrison Bros discussions. Also, for the record, I would like to try it for myself if I ever get a chance, not sure I will at the current price though.