greenslopes
Sometimes tower controllers are hesitant to line up aircraft if the airspace they are firing you into is black with aircraft. It's not a nice feeling to line up someone behind the landing, with someone on final, and go to Approach for departure instructions to which they answer 'call you back'. There's a lot more going on up in the tower than you realise sometimes, and it's not just about the aircraft you can see and hear in the immediate vicinity.
Cheers,
NFR.