You should also keep this in mind that all of USA uses 24 (+Ve) system not 48 (-Ve) system. Hence all the arguments do not really hold much grounds. these are two different systems, that’s all. the voltages had to be in the multiples of basic 2 V hence -48 V or 24 V, actually it turns out to be -54 V and +27V respectively. However there is one fact though, positive ground is more efficient then negative ground.