It's likely a combination of loiter time, capabilities, safety and availability. First you need something that can stay up for a long time so you either need a fleet of medium size devices or a larger device. Then there's the type; a plane type drone can't maneuver and tightly circle an area in a crowded downtown so to keep eyes on something you want a helicopter. Then however you run into issues operating that inside a city remotely, relying on GPS and you might have control interruptions so it needs to be able to safely navigate independently or at the very least stop and hover all on it's own.

It's a pretty tight set of requirements that might not have anything that fully ticks all the boxes yet as easily or fully as a human piloted helicopter. It's partially also familiarity and existing infrastructure. Most large cities have had some form of police helicopter service for a while so there's already support infrastructure and helicopters bought that won't all convert over to autonomous/remote operation and for those that don't it's much easier to follow an existing pattern than it is to pioneer something.