Are you really that dense? Helicopters cost big bucks to lease, maintain, and operate. Most cities and counties cannot afford them. A person can hear a helicopter well before it flies overhead. And because of its size and noise, there are severe restrictions on where it can and cannot fly. Only the largest metro police forces can afford to operate them.
Drones, on the other hand are smaller, better equipped with the latest survaillence technology, and are much quieter. And with each passing year they are becoming smaller, more efficient, and some are even equipped with stealth technology. But the biggest pay-off is that one doesn't have to be a certified pilot to operate one.