The reason that you see so many multicopters for the smaller ones is that they are much mechanically simpler, and thus cheaper to manufacture. All you need mech

The reason that you see so many multicopters for the smaller ones is that they are much mechanically simpler, and thus cheaper to manufacture. All you need mechanically for a multicopter is a number of independent rotors that you can vary the speed on. A traditional helicopter has a very complicated assembly to allow powering a rotor that can tilt forward, backwards, and to the sides, as well as change the angle of the blades. This is a lot more expensive to produce. It is more efficient, in a power to weight ratio sense, to have only a single motor, which is why you see this design on full-sized helicopters. But for smaller, cheaper drones, you don't need as good of a power to weight ratio to make it worthwhile, and the cheaper simpler design makes them easier to mass-produce and sell to amateurs or professionals on a budget. The things that probably have led to an explosion of them recently include electronics getting cheaper, and control systems getting better. Regular helicopters can be piloted by a skilled operator with some training. A mutlicopter needs a automated control system to keep it balanced. As electronics for the control systems have gotten cheaper, as well as MEMS sensors (accelerometers and gyroscopes built into ICs), these control systems have become cheap and easy to integrate into mass produced products. In addition, battery technology has been improving, as has electric motor technology. While previously only the single big fossil fuel engine was the only viable power source, as battery capacity has increased and electric motors have gotten more efficient, it's more viable to have small electric copters. Another thing that makes these more accessible to casual amateurs, and again makes mass production more viable, is that most people now carry around a flexible input device with wireless control capabilities. That means that you can sell devices without a dedicated controller, can do things like streaming video back from the drone, and so on, increasing the range of things that you can do with one. Other technological improvements also help feed into this. Having widely available, small, light HD and 4K cameras means that you can actually do something useful with these drones; back when to do any real filming you needed a big, bulky camera, they weren't as efficient for getting aerial shots, but once there are small, light, high quality cameras, it becomes feasible.