I'm gonna assume it's the limitations on time allowed to operate the mountain. As a business person I know there are ways to scale back operations to meet expected demand, and even if they do loose a small amount of money during the spring they would still stay open for branding purposes.
Economics. Cost to keep Mtn open and operate versus the number of skiers. I've also heard that since the land is leased there is are restrictions relative to the length of time the mountain can be open.
After saying the above I wish they could extend the season even a few weeks for great spring skiing!