They haven't expanded because it keeps the prices up and the stadium always full. The Utes have a 98% retention rate of season ticket holders because once they give them up they will be put on the back of the waiting list if they want to reacquire some. Stanford did the same thing when they rebuilt their stadium. They actually reduced their capacity from 85,000 to 50,000. Like I said the Utes have sold out 32 straight home games so demand for tickets is pretty high. One of those games was the last game of 2013 when the Utes had Colorado at home and had only won 4 games up til that point. Their strategy is working.
I still think they will rebuild the south stands soon and even add some seats but 50k will be as high as they will consider. When they rebuild that part of the stadium the Utes athletic department will try to determine how many fans would attend an average game and make sure they keep capacity well below that number.