some reasons I think...
1. old people, they ski there, alot of them own it, and its always been that way
2. mostly traversing but all around mountain travel, specially becuase booting on certain parts of the traverses makes the slope alot more avalanche prone, and alta requires alot of pushing in some places when its deep, steep to flat - steep to flat, terrain flattens out in alot of areas
3. skiers, if they are good, when they go down the fall line they push snow across the hill, boarders push it down, snow stays better longer, but becuase its so good it goes fast, alot of alta locals, and weekends(most of the time)...its called powder half and hour
but alta still kicks ass, the terrain has so much to offer