This is a good question. Rather than answering each part piecemeal, I think it's helpful to set up a comparison to Robotics alone.
Robotics (for general automation and control questions)
This is SE's catch-all for everything automated. The robotics field is so huge, it can arguably cover washing machines, elevators, coffee pots, and all of commercial aviation. Understandably, the robotics SE is therefore the home for any specific robotics branch which has an insufficiently small community to be self-sustaining.
Similar to the OP, it would be fair to ask if robotics would naturally be covered by Engineering and Physics, as well as Electrical Engineering. However, we all would recognize that Robotics, while it touches on all those fields, is none of them. It has special interactions which lead to frequently recurring problems, and this is where Stack Exchange's Q/A approach excels.
Drones/UAVs can be described in the same way. Better, it is a very large and relatively homogenous field, with millions of users who all interface in very similar ways with very similar themes.
- (almost) all drones are described by the same relation to 3D space
- (almost) all depend on an IMU
- (almost) all use GPS in (almost) the exact same way with (almost) the same antennas
- (almost) all are heavily automated through flight algorithms
- all depend on the same aerodynamic forces to fly.
In contrast, "Robotics" is a catchall which, frankly, is too diffuse to be very helpful to a targeted community. As a roboticist-- specifically an aeroboticist-- I don't find hanging out in Robotics to be very helpful, because the questions are too imprecise to my field. For example, it is very hard to answer a question about Kalman filters without precise knowledge of the application-- Kalman filters work really well until they don't.
"Drones", on the other hand, is unlikely to get diluted by questions about how to make a five-finger soft actuator, about how to sense bilge water levels, about addressing of sensors on an SPI bus, etc... When someone explores a new Kalman filter derivative, the community can bring a helpful amount of specific information which is relevant to the particular challenges shared across the drone community.
The upshot is that a Drones SE has a combination of purity and scale, which I would argue are the essential characteristics of a successful SE group.