Because people confuse being eloquent and articulate with intelligence. As a teen and a young 20-something, Rooney was not comfortable expressing himself, it was pretty obvious, and is probably an introverted person by nature who comes alive in his comfort zone (on the pitch), but when you looked beyond the form, it was quite evident he was a relatively bright person, who could be interesting in quite a few interviews.
I've not been particularly surprised at how well he's handled himself through this, or since he's moved into management.