Sooo... did they choose to uprez the PS2 version instead of the GameCube version because they were more familiar with PS2, and then choose to uprez the sequel at some later point? Because the sequel is on the Wii, aka the GameCube Turbo. Seems like it would've been easier to work on two very similar games rather than introducing a mess of broken PS2 code to deal with for no apparent reason.
I mean, if they can get a Wii game to run on PS3, then it should be essentially effortless to make it's same-engine lower-spec GameCube predecessor run as well, shouldn't it? Adding a PS2 port to a Wii port is doubling their workload. Even the PS2 extras were just "cutting room floor" material from the GameCube version. It shouldn't have been hard to reinsert them in a GameCube-to-PS3 port.
That's why I can only figure that they started this project in the laziest way possible, with just a port of the PS2 version. When it turned out badly (30fps @ 720p, with broken effects) they threw another game at the project as a "bonus feature", but if they had put that effort in from the start and went to the source, it'd probably be running a lot better.