I always assumed it's because 60 is a highly composite number (superior, in fact!). It's kinda the best number if you're ever going to need to divide. 50 is kinda garbage in that regard. :/
Another point in favor of 60 is there are 60 seconds in a minute and 60 minutes in an hour. If you make the frequency 60 cycles/second then you only need divide-by-60 circuits to derive 1 cycle/second, 1 cycle/minute, and 1 cycle/hour.
With 50 cycles/second you would need both divide-by-50 and divide-by-60.
It's worth mentioning 60Hz vs 50Hz distinction has ended up having a knock-on effect on framerate standards because of early TVs using it for timing, not to mention market segmentation because different products had to be manufactured for the US vs European markets.
The number sixty is highly composite maybe because it's a multiple of three? In which case, I can see why Nikola Tesla liked the number three or multiples of three.
So he can do exploratory electrical science and analysis with flexible cases?
