Well there's two different types of resorts.  The one's that live off the local skier traffic and the one's that are tourist draws.  When the sun comes out and it gets warm, people really do forget about the mountains.  In the last two weeks I've had 4 different occasions where the person I was having a conversation with said "you're still skiing?"  or "they're still open?"  and even on resorts that are just there drawing off locals it still has the same effect.  People would rather golf or bike or work in their garden.  There is a small number of people that would actually ski (small number looking at the big picture.  If a resort gets 4000 skier visits a day and only 300-400 in the spring it isn't going to be profitable very long).  Look at how my former resort (when I paid for my seasons pass and went up whenever I could get out of work), Mt Hood Meadows does it in the spring.  I'm sure they've started the discussion up in their blog already (which is very interesting to read, it's on my recommended reading list...).  They give people an ultimatum.  If we get a certain number of skier visits this weekend, we will be open next weekend.  If not, then we shut down for the season.  And they have alot of snow this year, but the interest in being on the snow really takes a dive after...  I want to say late april but I'm pulling that out of the hat...  for them.
As well, you need to look at what a resort has financially budgeted for the season.  Lets take the grooming department.  Sometimes you go over your budget for the season at the end of march.  If the resort was to stay open, you would need to make enough to pay for the gas, operators, maintenance and such.  Same with every department.  If you start eating into the bottom line then you can't do the improvements a resort may or may not want to do over the summer for the next season.
I hope my rambling may have answered anything?