People who grew up with it aged to a point they can now pay for things they had growing up. It's what happened with all previous retro game markets. They were cheap and affordable until the people who grew up with them wanted them again. Same will happen to 360, PS3, and Wii, DS soon in a couple years. Just make sure to get the hidden gems you want before they do go up.
It was never this way with past gens, with Mario 3 for NES ever being like, $50-100 (at least, over a decade after being available for a high msrp) just because people can afford it now. Even if they can afford it, no one seems to want to do their homework, and just agree to pay for way above original msrp/typical going rate.
It's moreso a recent trend, and although some GC were always pricey due to being obscure, it just EXPLODED as of last year.