Heavenly has to get a lot of natural snowfall or make a lot of snow to open much terrain due to the HUGE rocks that are everywhere. Kirkwood and especially Northstar have much more grassy slopes that require less snow to open. The TRPA regulations on the Tahoe watershed side prevent much movement of dirt and rocks, and it would be a huge undertaking even if it was allowed. They are experimenting with filling in some of the voids between the boulders with cut logs and branches to improve the situation a bit.
I don't really feel like Mammoth is a viable option for most of the skiers that the Tahoe market serves. It's 140 miles further to drive, and most people aren't that dedicated even if the snow is better. We plan to go again this year later in the season, but it's not because the snow is better -- it's to meet up with friends that live in Southern California.