how come in videgame previews, they never try it on the ps3 first

It's the version that the developers brought with them, maybe it's the more polished version at the time.

look at mercenaries2 they brought the ps3 version at e3 because they said it was the closest to being the final product

not all of the time, this e3 I was surprised when they showcased a lot of games in the PS3.Tlahui

Same, I was watching G4 and they were showing Ghostbusters and the guy played it on the ps3.