I agree, I think it is because of two factors...the huge install base of the PS2 (100 mil systems vs. 25 mil systems for the Xbox) means there are more people looking for it, and also a lot of people prefer the PS2 controller. Plus, there is no Live play on the Xbox version, so that is not a factor.
Personally, I have the DC version which seems to be the best version (and it's the cheapest!)