I remember reading somewhere that it was being delayed so Capcom could add an extra game. So at least it's for a good reason. I'll see if I can dig up the article somewhere.
EDIT: I found the article.
Mega Man Anniversary Collection GBA gets delayed, but also another game? EDIT AGAIN: I was poking my nose around the Planet Mega Man forums, and it looks like the addition of a sixth game is pure speculation.
See for yourself. People are gussing it could be anything ranging from the WonderSwan game to the Game Gear game, to Mega Man Soccer.
Apparently the source for this is a preview in an issue of Game Pro where they same the cartridge has six games, including a "hidden bonus game". However, a little later in the preview, they talk about the anniversary collection that was released on PS2,
Xbox, and GameCube, so this information is spotty, at best.