Urban Meyer made it hip to be a Ute fan and he also backed it up with wins against the Y and by winning a BCS game and having a number 1 pick in the NFL draft. It's been downhill since that point. Prior that point, RES struggled to fill their 32,000 capacity stadium... I know this because My uncles and Dad were season ticket holders for years so I remember the day when Ute fans were extremely minimal and I'm telling you that was as recent as the early 2000's.
It's a combination of several things but giving the youth of Utah something to find remarkable with two BCS wins and the PAC 12 entrance along with dominating the state rivalry has went way farther than alumni could ever take it.