Controller boards and multi-axis sensors became cheap, powerful, and easy to work with. Companies then sprang up with pre-made kits that weren't too expensive.

Controller boards and multi-axis sensors became cheap, powerful, and easy to work with. Companies then sprang up with pre-made kits that weren't too expensive. If you're interested in seeing the start of this craze, check out the Wii Copter. The sensors in the Wii controllers, and also cheap boards like the Arduino, made it so any hobbyist could build one.