As always with the D-rays, I think the name of the game is log-jam.
Obviously with Gathright gone...and Hollins not exactly being great it's a tad easier to bring him in, still you have to figure they'll probably wait to bring him in until they need bodies to fill spots on the team if they trade for prospects.
The other thing to keep in mind I think was the arbitration clock that they were looking to not get moving by giving him a call up before needed (not sure how that whole system works, I just recall it being a reason why he wasn't brought up at the end of last season).
One more minor detail, which is just speculation on my part is that from what has come out in the news (which isn't really reliable) he comes across as having a bit of an ego (I can't say big head here any more because then the "S" word comes out).
So, if you had a very talented player in the minors who didn't need to be called up right away, and seemed to think he was better then everyone else...wouldn't it make sense to hold him up down in the minors in an attempt to teach him humility?
Just a thought. Besides, if you can teach him humility it may help them sign him longterm...because let's be honest, right now Tampa Bay is a blip on the radar for Delmon who I suspect already has himself starting on a big market team down the road.