Because every player has a different strength and weakness. And admittedly, Mata's strengths are more of a specialist rather than a generalist, as in he is really good when players around him can play his way, but not good when others have a different playing style, if you get what I mean (and if you care to read all the posts, all of us have literally been saying this). It's like asking Messi to play with Sam Allardyce's players, and then wondering why he isn't scoring 50 odd goals a season.
With Mata, you can literally pin point games where he'd do a good job, and where he won't and you'd be right in 90% of the cases- which is a great thing for a manager. And I've no idea why he's being compared to Bruno or KdB - all of us have repeatedly said that he isn't as effective as Bruno in most of the other games.
And around your argument that Bruno can raise others game while Mata can't - it depends game to game. In games such as the one vs Newcastle, Mata's play was more important as it was his pass and move that caused them issue, but against say a Brighton where the defence is already quite open, Bruno's directness will help the team more.
FYI: That's going to be my last post as I don't see this argument going anywhere, so we'll have to agree to disagree.