Guess #1: Maybe the games are simple and inexpensive to make. They’re not of the quality a title for the DS/PSP is expected to be. Essentially they are just trying to make a quick buck.
Guess #2: SE is trying to expand a “new” platform. Maybe they’re hoping that more people will buy higher end phones which will allow for more games to be sold in the future.
Guess #3: Inevitably your cell phone will be a computer, mp3 player, and gaming system all in one. Maybe they’re gaging public reaction or just trying to speed this along.
