[fnordfnordfnord]

I agree with other posters that it has been the culmination of several things.

Batteries with high energy:weight.

MEMS sensors (thanks, nuclear weapon stewardship programs) http://mems.sandia.gov/

Cheap, open-ish, powerful SoC microprocessors, and the community of hackers (lots of help, and easy tools that lowered the learning curve).

Development of better "linear" circuits like advanced brushless motor controllers and chargers.

Growing robust hobbyist electronics tinkerer marketplace.

Maybe cheap 3D printing too, making prototyping easier/faster/better/more fun.