When the cabin is ready to depart (door closed, jetbridge retracted) but we don't push back right away it is usually waiting for the final bags to be loaded and/or waiting for our pushback clearance.
The ramp personnel can see the status of checked bags and cargo for the flight on their wireless/scanning devices. They know if checked bags are missing and will wait without the guidelines that have been established for them. We don't have that information up front. We just see that a cargo door is still open.
Depending on the airport, our pushback can be delayed by other traffic and congestion on the ramp. DEN is one of the best with all of the room they have which allows us to push straight back without getting in the way of arrivals or departures from nearby gates. EWR and LAX are the worst. Single alleys mean that a pushback will block incoming arrivals as well as other pushbacks.
Less frequency, we are still preparing the cockpit (when we arrived late from our last inbound flight), are dealing with a maintenance issue (waiting for the paperwork to come over ACARS), programming an ATC re-route, or modifying our flight plan with dispatch.