Because the airport operations basically come to a halt during VIP movements. They started up the arrivals not long after AF1 arrived, but departures were stopped until Marine1 departed.
Oh, I meant intermittent communication with AF1. It's clear that the other aircraft were just told to monitor tower (this is evident on the ground feed). On other archived arrivals of AF1, it seems that there is simply more being said being the two. Minor observation, but I may be wrong anyway with this one.