Blame all those cheeky buggers who built machines powered by steam engines or water turbines. They basically built their machines to run on whatever was available. Got a waterwheel doing 1000RPM and you need a big saw attached to it? Your saw will be running at 1000RPM (~16Hz).

So many crazy machines were built before there was either a need or an effective way to standardize anything power related. Keep in mind that many kinds of manipulations of electricity are easy and cheap now, but that certainly wasn't the case when people started converting from steam/water to electricity!