Because quadcopters are naturally unstable, so you need complex electronics(and sensors and electric motors) to control it.<p>You can make helicopters very stable just with weights in the rotor, like any RC fan knows.<p>Until very recently the inertial motion control complex elements were very expensive.<p>Accelerometers in cars and gyros in smartphones later had made those sensors inexpensive. The iPhone was released in 2007, that was 7 years ago.<p>Every air device carries gyros inside, but for controlling the plane they had to give you a digital signal, it is not enough with the artificial horizon sphere.<p>The simple helicopter design has been mass produced for decades(and wars like Vietnam subsidized them more), and it is well tested and reliable. Any new design has to compete in price and will have to iron lots of bugs at first.<p>The second most important reason is that you need to use electric motors as normal heat engines could not respond as fast as needed.<p>So in order to make a quadcopter you need to generate electricity onboard, which means a big hybrid engine.<p>I see quadcopters as the future, but in order to compete in price, they will have to evolve from small UAVs.