Why did X360 Borderlands outsell PS3 version 3:1 ?

[disolitude]

360 userbase is based on games like borderlands. Back from xbox days, online and coop were big part of the Xbox DNA.

PS3 users had uncharted 2 which looks to be more than enough for their coop needs.

Borderlands is my GOTY BTW...right up there with Left 4 Dead 2.