not sure what the whole story is; never tried to research it.
However, most aircraft rarely run on batteries for extended periods, and the power converters have to be sized for the alternator outputs, which, for MIL-STD-704 are quite ragged. Additionally, the overall voltage range of both mil specs cover 24V, since the standard voltage range is 22V to 29V.
Most airliners run on on-aircraft auxiliary power units (APUs) while on the ground, or directly off ground-based APUs.
On many aircraft, the batteries don't even go to all the electrical systems, thus making it imperative that they can reliably run on 28V from the APU as opposed to battery that might not even be there.
TTFN
FAQ731-376