It was Shinji Mikami, the series creator, who pushed Capcom to let him make his series exclusively for the GameCube, since he realized that the PS2 was a piece of crap. Albeit a great selling one, which a lot of people (including Mikami) have made good games for.
His bosses went along with the idea, but they decided that they would make RE Online and some other "spin-offs" exclusively for the PS2, without any input from Mikami, in order to keep Sony from getting upset with them (since Sony's practically a dictator right now).