Electronics Engineering Question:

You need some sort of AC for power transmission and distribution inside of aircraft. Power companies use 50/60Hz because any higher frequency results in excessive inductive and capacitive losses, given the many miles that have to transfer power over. But a higher frequency gives you lots of advantages in transformer and motor weight, and over the short distances in an aircraft, the inductive and capacitive losses are not excessive. So overall, the higher frequency saves weight, and for aircraft, that is important. Like all designs, the choice of 400 Hz is a compromise between losses and low weight.