That's because he didn't really have that much competition in that position
He is so much better as a midfielder.
Not so much better, but better for sure.
It was still a very very good decision to play him on the left wing back then, because otherwise we'd have had to play Peluso or some other squad player, and there wasn't any player available on the transfer market who'd be as good or better as Asa for any price we could afford. And Asa of course did very well as a wingback too.
Now, we've got Evra & Sandro instead, who are both probably (slightly) better than Asamoah in their position, so we can utilize him in the role that brings the best out of him individually.