Because electric motors are much easier to control. An internal combustion engine is not so easy to make it spin with a well-defined number of revolutions and quickly change this parameter. For several propellers, you need to set the minimum differences in rotation speed to make the aircraft move in one direction or another or hang in place, and with the help of electronics and controllers this can be done quite subtly and clearly. And doing the same by adjusting the fuel supply is, firstly, difficult, and secondly, it is impossible without a noticeable delay between the "gas" and the change in the propeller rotation speed.
https://geektimes.ru/post/257940/ ___ _ _____ _ _ _ ____
_ _ _ _ _____ _ _ _ _ _