I understand that historically it was 16 2/3 Hz and power transfer from to the 50 Hz grid was done via 1:3 mechanical coupling with a motor and generator. That was a ratio easily achievable with gears.

Nowadays with switched power supplies, this is not a problem any more. Keeping track of 16.7 Hz seems a little easier. Imagine building a numeric display for a power plant operator to see how far off you are.