I suspect 50 vs. 60 Hz is mainly related to decimal vs. pre-decimal (multiples of 6) thinking and derived from the generator RPMs. A two-pole 50 Hz generator runs at 3000 RPM, a 60 Hz one at 3600. Europe was already pretty much metric and decimal by the time electricity came around, except for the UK.
Was the 120/240 V 1-phase developed alongside Edisons 3-wire DC? It'd seem more logical that it was a reaction and an attempt to use existing DC mains and existing equipment (anything that'd run on both DC and AC) during the conversion from DC to AC, at least to me. That's just an educated guess though. The only thing I do know is that all mains voltages in the world were likely derived from the operating voltage of series-wired arc lights, i.e. multiples of 50-60 V.
