Not convinced he possesses the level of ability to play week in week out at a club like United and he might have a questionable attitude. I feel for him a bit though, he joined the club at the wrong time, which wasn't his fault. I think Fergie would have given him an opportunity, he made the signing and could afford to take gambles on raw talents.
Number of things really. Fergie signing him and then Zaha not being able to work with him might have played a part in Zaha's apparent questionable attitude, quotes from Holloway prior to the move suggested his attitude was very good. Ultimately I think the move was too soon for him, looked exciting in Championship but very raw. Other clubs' interest may have forced Fergie to sign him sooner than he'd have wanted to.