Yeah... same thing with the new Crash game, I believe it also sold well on the Cube, but the sequel has only been announced for the other two systems. It seems as though the third-party developers are ignoring the previous games' sales as well as the fact that the Cube had an excellent holiday season and many new owners will be looking for games to buy.
Not that either game will necessarily be worth buying (as the last Crash game was nothing great), but many third-party developers seem to be shooting themselves in the foot in terms of available audience.