Damn you shelf-fillers. Driving up the prices for everyone.
I think more than that, though, is just the general popularity of the hobby. 10-15 years ago, I could go to the local used record shop, choose from a whole case of NES, Genesis, SNES, and Atari games for about $5-$20 a pop. And that was common.
Since people like Angry Video Game Nerd, Classic Game Room, and Storage Wars has come along; it has caused a spike in popularity for the hobby that just used to not be there. You can't find most of those carts anywhere for a decent price anymore, it seems (except maybe Atari 2600).
I distinctly remember showing people my collection many years back, and a good majority of them would say things like, "Why play that crap, you know PlayStation and Dreamcast have way better graphics" (which I did own at the time, too). But now, people are legitimately impressed by it.