I think it has a little to do with the economy.
Probably the main reason is that the average person not wanting to spend these high prices for the RB drums. When they were $1500 - $2500 for a nice 4pc a few years ago that price was easier to justify. But when you see a lot of them going for $3500 - $6,000 and more most people think a little harder or even just can't afford that kind of cash. And some think that the drums are no way worth these high prices to begin with. So when all these people are out of the market that narrows potential buyers down considerabley.
Myself.........I think that most Vintage drum prices will continue to go up. I've heard it said recently that "Vintage Drums are just starting to catch up." Look at some other instruments - guitars for instance: Vintage drums are no where near some of those prices. And to compare......there are way more parts to a set of drumsD' Drummer -
and better looking IMHO.