We are used to thinking only in terms of a 60 cycle AC 110/120 volt electric system. The rest of the world has different standards. As systems grew over the years, additions to every countries systems had to be compatible with the old system and this locked them in further, as it did with ours.
Similar situations exist with things like railroad tracks. The inside width between the rails is 4'8-1/2" in the US and England, one of several metric gauges in continental Europe and 5' in Russia. This was done to keep other countries equipment out of another country and the other countries that were colonized by European countries normally took the variation of the mother country, as we did England's.
The same thing happened with electricity but railroad tracks are easier to see and visualize than electricity's variations.