But why 115 Volts?
400 Hz up in air BUT not on ground makes sense because of the inductivity losses. High frequency alternating current tends to have skin effects, proximity effects et cetera. When you have a plane at most 80 m long, the losses can be dealt with; in an extended network on ground you must have much lower frequency. High frequency limits the weight of transformers and other coils.
Now, as for voltage - the aircraft DC is said to be 28 V. Why this - not 24 or 36 V?