It only came out on Friday I think... and this isn't like Japan where a shitload of sales happen on the first day. Hopefully next week it scores a decent position.
I wouldn't be surprised if the PS2 version overtakes it eventually, but initially I'd imagine it'll be cube sales. The PS2 media here are giving it a pretty low priority, whereas it's quite literally the only Cube release of interest since Resident Evil 4 arguably... it's been cover story on all the cube mags at some point in the last couple of months.
Me and a couple of the guys at work bought it on Cube anyhoo. Gonna go play it in a minute actually..