I dug out an emu for the GP2X and I realized how ass ugly and limited many GB games are compared to their NES counterparts. At the time, it didn't matter because it was the only available portable gaming choice. Some games did manage to pull nicer graphics (e.g. SML2) and I like the chiptune music most titles have... I still play the odd round of Picross.. but I feel most games didn't really age well.
Or maybe it's because the Wii is not a portable console, and they don't want to blur the lines with the existing GBA and DS products. I guess it's the same reason why there are no arcade games available, they don't want to open the can of worms. (and also, these games are too easy with unlimited continues... which is why Neogeo in MVS mode gives you 4 credits).
[download a life]