The fact you just rattled off 3 players that we already have as cover for that position is probably a big reason. Regardless of whether he is better than them or not.
Do we really need over a quarter of our playing roster used up on 1-2 position/s on the field?
It could be just unfortunate timing for Tim and the 'Nix. If he was available this time last season he probably would have been signed ahead of Hicks and Riera. (Riera might not have been signed at all). Looking at our strikers, I would prefer that we get the best possible player for that position and then if that means we have no more visa spots available for an AM then Tim I think could come into the reckoning again (though he might not be available by then).