honestly, because cloud kicks his ass. also vincent was only a OPTIONAL character.
barret diserves a shooter. even though the story wouldn't have been that good.
Vincent's story wasn't developed a lot in FFVII so I think it was a good idea to put him and not Barret. What kind of story could they have made from Barret? We already know his past and all.
A shooter with Barret would have sucked. Also why are you saying Cloud kicks ass? You think Cloud should get his own game? Cloud was already the main focus in FFVII and AC, and that's more than enough. Vincent is also a lot cooler than Cloud, IMO.