Overall there is a cultural movement going on right now (and for a few years back) that puts a premium on vintage and "authentic" consumables. Bikes, guitars, you name it.
I'm 34 and my generation is in the middle of it, I think - though it started in the 80's. It used to be about finding cheap, cool older items that were a bit unique but available. As our incomes rise and the nostalgia sinks in deeper and deeper so do the prices on these goods...it's part supply and demand and part romanticizing of the objects.
As it is often spoken of here - everyone is looking for a cheap but ridiculously cool bike to flip or restore. If people were simply looking for a tool like a bike to ride or a guitar to play the prices would be much lower. As it is, we are all looking for that great find and thus the prices keep climbing - vintage bikes are just more popular now than they were a few years ago. It will ebb eventually but who knows when...