I assume it's because Texas actually has a long history of telling the state to go fuck itself regarding funding because the university can make enough money on it's own to stay afloat. For example, the state wanted to cut funding when integration began and Texas started admitting black students, and the University called it's bluff and went a few years without state funding and was just fine. Being able to handle your own banking keeps both the state and the BOR in line.
This is actually precisely the reason Rick Perry was able to fuck with A&M so much. When students at A&M massively protested his 5-point program (in lock step with University of Texas students, actually), A&M had to fold because they were in debt, while Texas got to successfully get around his 5-point program. Very honestly the only time I ever felt sorry for the Ags, cause the governor was fucking with actual student education there against their will.