He plays in the wrong position
As Vidal’s displays for Chile at the 2014 FIFA World Cup proved, his move to Juventus has seen a transformation in the player from the more box-to-box minded man at Bayer Leverkusen to an out-and-out attacking player.
The systems, usually playing three at the back, that are employed by both his club and national sides, not to mention the presence of Paul Pogba in Turin, allow Vidal the freedom to play in this position.
At United this is unlikely to be the case – although Louis van Gaal has toyed with three at the back during his spell with the Netherlands it does not really fit the squad that he will inherit at Old Trafford.