How about they're older, wiser, and some of them richer, and while they may do the odd mini here and there for characters they really like, they'd rather drop dead than get back into the monthly deadline grind of a major Big Two book, the warring with editors etcetera. There are people in their 60's and 70's.
And of course Chris Claremont lost whatever writing ability he had about 5 years before he ended his 15-year X-Men run.