the reason why FT get's one i because Hiro, it seems, has enough free time to write 2 manga at once, they also gave spin-offs to other artists to write which means the whole of FT will now be even more of an inconsistent mesh (and that i why I will be sticking to FT and maybe the monthly FT prequel from Hiro)
unlike Hiro, Oda doesn't have the time to do 2 manga at once, Oda gives his all into his original One Piece
as for spin offs, please shueisha never let anyone else beside Oda work on One Piece as long as Oda is still working on the original (one and only) One Piece