Beside money, fame, lack of inspiration, evolution of sound etc. the biggest reason I think it's age.
This is something that I have witnessed happening in the audience too.
When you're young you have all this energy inside you, you want to hear something powerful, crazy, fast.
When you're getting old you want to settle with something more mellow.*
I'm sure everyone knows people that used to listen to psy, but now they feel too 'old' for that.
That's the case with the artists too. When they first produced the just want to make something intense and crazy, but after some years they want to calm a bit. That's the only reason I can think for example that none of the old school artists evolved his sound in something more fast or crazy, but almost all of them they went to the full on/prog side which is a mellow one indeed.
Of course there are more money there, but I doubt that all artists think that way.
*BTW that's not the case with me. Personally the older I get the more psychedelic sound I want.