USC has way more national championships and Heisman winners. UT is in the top tier, but there are lots of schools in that top tier and none of them are always "at worst a top 5 program at all times.
There are a few teams who have much richer traditions than UT, your example of USC is a good one.
What puts UT into the ranks of the underperformers is their status as the highest income athletic department in the NCAA along with sitting in the middle of arguably the richest HS recruiting state in the nation. Historically UT has been able to almost take their pick of Texas talent leaving other schools to fight for the rest.
Even with a number of other BCS level programs in the state UT also is the most dominant college program in the state for media coverage and fan support.
Mack Brown has won a NC there and won a lot of games but you could argue that the NC came as a result of having Vince Young and with the talent available to him and the schedules they have played he should have won a lot more games.