Because Rooney has a more expansive footballing mindset than Berba. Rooney slows the game down for a reason, while considering the options his sublime talent & imagination gives him; considering his undoubted skill, in contrast Dimi is all-too often content to merely lay-off an easy pass. With his gifts, Berba should be providing more, in a play-making sense; there's no good reason why he could not attain the heights of even a Cantona for us.
(I should mention that I'm actually a Berbatov fan. But, sadly, I just don't think he's truly risen to the occasion of playing for United, despite last season's goals. I mean to say that he could, with a little more imagination and effort, be one of the greatest stars the club has ever had).