From an forum titled "The Wings of the Web" on Airliners.net
I found a conversation asking the same thing and read this:
Larger aircraft adopted 24/28 (battery/generator) reduce the weight of
the wiring system and components. Since work relates to the Watts it
consumes (electrical power) and Watts are a product of voltage and
current, the higher the voltage you use the lower the current
requirments and the smaller the components (to an extent) and the
Some aircraft (like the SAAB 340) have a 48 vdc starter that uses the
2 on-board 24 volt batteries and connects them in series during a
start. So even more power is available with relatively small starter wires.
Most larger aircraft use the battery for starting the APU only. So
there is no need to go higher in voltage. But since AC power is
plentiful, they use a battery charger that is powered from AC. It
still only puts out 28 VDC though.
Here is a link to the orginal forum post.
I do realize that your question was directed towards spacecraft specifically, but since they are still in the world of aviation, there tend to be a lot of shared components. Because of this, I assume that the space industry's reasoning would be very similar to the aviation.
I hope that this quote, helps,