I'd assume the following reason: Planes are the "easy" default, clearly legal to operate, well-understood and most people don't think twice if they happen to see a Cessna somewhere.
Drones probably have legal issues, big ones (range, payload) are also quite expensive to build and operate and require more specialized skills (unless there are tons of ex-military drone operators to be hired cheaply?). Would also probably attract more attention.
Blimps have been proposed and tested for all kinds of surveillance tasks, but you'd get the main benefit again if they were autonomous. Seems to be in development, but not there yet.
Drones probably have legal issues, big ones (range, payload) are also quite expensive to build and operate and require more specialized skills (unless there are tons of ex-military drone operators to be hired cheaply?). Would also probably attract more attention.
Blimps have been proposed and tested for all kinds of surveillance tasks, but you'd get the main benefit again if they were autonomous. Seems to be in development, but not there yet.