There's another good reason for the 400Hz freq. and that is to provide greater DC quality (smooth and clean) as opposed to 50Hz. Weight and Volume are the other main reasons.
GD&L
Full-wave rectification on a single-phase source yields a ripple frequency 2X the line frequency. A three-phase rectifier gives ripple freq. 6X the line freq., and yes, that's a lot easier to filter to pure DC. This further reduces weight and volume.
