I've been thinking about this, and Utah has an unusual competitive situation for day skiers. The entrances to BCC and LCC are a few blocks apart, and once you are in one you have Alta vs Snowbird within a stones-throw and Solitude vs Brighton right next door in the other canyon. And you go right by ski stores that sell discounted tickets on the way in.
In contrast, while the Colorado areas are close to each other, they are not that close. Changing your mind at the last minute is going to lose an hour or more of your ski day. The year I wound up paying full walk-up prices at Breck I grumbled but did it.
The fact that A/S/S/B allow the stores at the foot of the canyons to sell discount tickets is another indication that there is real competition. Most places you cannot do that. (For example, discount Park City tickets are not available near PC. You have to get them before you get close.)
Edit - ok, I am starting to fall in love with my own theory. For season passes, the geographic constaints are looser and the Colorado areas are competitive with one another. So we circle back to the original posters question, rephrased as why is there so little real competition for SLC seaon passes...