Have you ever noticed you can answer pretty much any "why" Broadway question the same way?
"Why are some Playbills black and white?"
"Why do tickets cost so much?"
"Why is 'Rent' closing?"
The answer is simple: $$$$$$$$$
'Rent' has done extraordinarily well existing as long as it has, but it is not existing well enough any longer to let it become a full-on dead horse that requires flogging. Any extra life that has appeared this summer, from cast and audience alike, is because the end is in sight and they're making the most of it while they can.
Next dumb question?