For airplanes, lighter is better. 400 cycle operation allows smaller magnetics such as transformers, syncros,  alternators.

At the same excitation level, a transformer can be smaller by 60/400's - almost. Actually, the ratio is less. Transformer weight increases only by the exponent .75  of transformer power. So, the smaller 400 cycle unit will be penalized by this relation to size, but the weight reduction is still very large.

The max output level of an alternator is dependent on speed. A 60 cycle alternator can turn only 3600 rpm - or for more poles, 1800 on down.  A 400 cycle alternator can be poled to run at a practical maximum such as 12000 rpm (4 pole) and have a much higher power/weight ratio.

Thank you bzz41 !!