Ummm because he isn't big enough and isn't a sure enough tackler? (insert the Gibril Wilson comments here)
The guy is a cornerback and that is where the coaches feel like he accels.
I'll give you the size thing even though he is taller than Bob Sanders and only a few lbs. lighter, but to say he isn't a good tackler is absurd.
Allen is one of the better tackling DB's in the league. Tackling has never b een his problem. Now, catching a football......that's another story.
Not a chance.