Because Mata rarely does that - Rooney has a poor first touch, and it is something I find it weird that commentators don't pick up on.
Rooney was playing further forward, with his back to goal, while being tightly marked by centre backs who are bigger and physically stronger he is. It's hard to bring the ball under control in those circumstances.
Certainly harder than receiving it in midfield, where there's a bit more time and space.
That's probably why when Rooney dropped deeper, nothing got away from him. And why during his period in midfield, his losses of possession per game via a bad touch
compared favourably with Aguero's, Sanchez's, Hazard's, Costa's, Ozil's, etc. That's a fact borne out by the stats.
Speaking of Aguero, I thought he did nothing today apart from the goals.
That's a pretty big 'apart from' though, and is really besides the point. He is in that team to score, and he did, twice.
The point i'm making is that it's not only Rooney who can look a bit 'meh' while still contributing goals and assists. But Rooney, because there's a greater focus on him, gets far more harshly criticised for it.