Because he's a weakling. He's full of himself and good at nothing really. He has no work ethic, no grinta, no determination, the opposite of aggression unless he can break a leg. I absolutely dislike how he has such a poor tactical sense.
Sure, he's decent at infiltrating and showing up in the box. Thats about it.
- - - Updated - - -
Guardiola moved the best defensive midfielder of the current decade to CB 'cause he's not a playmaker'. Dont need to look further then that.
Javi Martinez is an awesome CDM, but Mr Cancer decided hates the notion of CDM so put him at cb.
What did we saw vidal do most of the time against juve ? Hanging between the $#@!ing cb's. Vidal. $#@!ing vidal.