I've never been terribly interested of Killer 7 before, so this really couldn't mean much less to me. It hasn't really been hyped up all that much, really, so I dobut that it wouldn't have done as well as a Gamecube exclusive. It makes perfect sense from a marketing standpoint - a new franchise should obiviously be on the console with the largest fanbase, which would increase the number of early adopters, more sales, and then, increase the chances of seeing a sequel to the game.
However, this hopefully doesn't apply to Resident Evil 4. It's the game that people are really interested in right now, it seems, and considering that Nintendo has a deal going with Capcom to keep the Resident Evil titles exclusive (most likely by forking over a large sum of cash) and because it already has a large fanbase from the previous two Resident Evil games on the 'Cube, it makes much more sense to keep it exclusive for now. Another problem would be the graphics - we all know how amazing this thing looks, and the PS2 simply could not handle such a thing without being toned down - and that would hurt the game, seeing as how one of the major appealing factors in the first place is the graphics of the game.
I'm glad that they're keeping it on Nintendo, though, even if it's not exclusive. After all, we're assuming that it was originally planned for it, so it makes sense to not scrap all of the work done in favour of the PS2 version; big waste of money there. I don't think that it was really going to be a system-selling title in the first place - more of a game to hold off the Resident Evil fans while they await eagerly for the new installation in the series.