It can't be all down to backwards compatibility. The Playstation 2 and 3 are baackwards compatible, too, but most PS1 games can be picked up for dirt cheap.
Something different about the markets.
I never cease to be surprised that Tetris despite being the original pack in and--according some statistics I've seen--one of the five most common video games in the world, still costs $15 most places.
As for why Nintendo didn't include Gameboy/Color emulation in the DS. One problem is that Gameboy and GBC carts require +5v while the DS operates entirely at +3.3v, so emulation alone wouldn't have been able to make original Gameboy carts playable, some hardware changes would have been necessary. It still would have been possible, but 1) it owuld have drained the battery pretty fast and 2) Nintendo were obviously trying to distance the DS from the Gameboy line to a certain extent. Though they hedged on that last point a bit by including GBA compatibility, of course. I'm glad they did and I sure wish they'd included original Gameboy support as well, but at least I can play most B&W Gameboy games on my DS using Goomba.
...word is bondage...