Keystone started night skiing waaay back in the day (I think '78 was there first year with it). I think the main reason nowhere else has added it is the huge cost of getting it going, especially at large areas. Night skiing works great at small areas and Keystone is able to shut down everything but the frontside. But everywhere else, you've got to put a lot of money into lights, lift ops, ski patrol (imagine sweeping at Vail, let alone in the dark), etc., etc. Plus it makes the scheduling for grooming and snowmaking much more difficult. Also, there has to be the demand for it. SolVista used to have nightskiing but they weren't turning a profit with it so they stopped and just do it once a month now (as of last season). Keystone doesn't lose as much because they are already running the Gondola 21 hours a day for the world class restaurants on the mountain (Fondue der Chessel and The Stube).
"The world would be a better place if everyone kayaked."-Brad Ludden (Valhalla)
"You only get one chance to run a drop blind."-DD