1. Snowbird has more in-bounds terrain that is avalanche threatened by out-of-bounds areas than Alta. e.g. Baldy and American Fork Twins pose avalanche threats for in-bounds skiing (while it's still an issue at Alta, it is less so).
2. When Gad 2 and Mineral Basin areas are open, there's a great deal of additional terrain and lifts to patrol, groom, sign, maintain, etc. Why spend the money on staff to do all that when most of the skiers you're getting right now are locals who paid by the season rather than by the day?
Take a look at what Jackson Hole is doing this December.