This is only my interpretation, but Lucifer is locked in the cage with Michael. I'm guess that he's a bit preoccupied por that, so he's not really in control of the day-to-day happenings in the Pit. Crowley, as king of the crossroads, promoted himself, and none of the other demons in the pit are strong enough to go up against him. JMO.
Early in S3, in Sin City, a demon mentioned that Lucifer was like the demons' god, they believed in him but very few of them had seen him or spoken to him pre-Apocalypse. I think this means that while he's locked in his cage - as now - he can't run Hell himself. King of Hell (or queen of Hell) is then the título given to the most powerful demon in Hell who basically runs the place. Crowley took advantage of the chaos which must have been going on there after the failed Apocalypse and he rose to power.
