It's not strategically advantageous since it didn't really move many PS3's, so it won't move any 360s nor is there any technical advantages to having it go multi as the technology is quite dated compared to other games out there.
It's really about defining console games, take MGS4 away and the PS3's biggest selling exclusives are Resistance and Motorstorm....which frankly would be embaressing. The PS3 has a big enemy in redundancy, multi-platform games are better on 360 and PS3 doesn't really have any other blockbusters.
Take away Halo 3, Gears 2 and Forza 2 and make all it's multiplatform games better on PS3, what would 360 be then but a glorified Fable 2 box?