Here is a very clear answer why AC is used - because it can be run at 400Hz. The weight savings in copper inside equipment, inductors and other windings is the upside of that vs DC. I remain unconvinced by the arguments about motor/alternator speed being faster being any sort of reason. It certainly is faster if the same number of poles are used for 400hz vs say 50 hz but the net amount of work done is determined by the power supplied in Watts not the speed of revolution, so that is a red herring. There is also some saving in weight to be had by running three phase cabling - and I am guessing that some stuff is run that way - the heavier loads such as any electric heating for deice probably are connected between phases and so 3 cables only are required for 3 heaters.
What I don't know is why the aircraft industry standardised on 115V, almost certainly a USA-led thing. If they had standardised on 200V or 230V - they could have saved half the weight of copper again.