In the past, the mains AC frequency was a cheap and accurate way to keep track of time. A lot of devices relied on the mains frequency.

Why does your modern day LCD monitor have a 60 Hz refresh rate? It's legacy from 1950's vacuum tube televisions that synced their beam with the mains voltage frequency.

The AC frequency used to be very inaccurate, it was only through a lot of work and legislation that it was made accurate, specifically so that electronic devices could use it to keep time.

Nowadays the electric companies want more leeway in how they run their grids and would love to have the frequency be less accurate, trying to keep it accurate actually costs a lot of money, since now as load changes on the grid they have to make sure to spin up new capacity. As the load increases the frequency goes down, as load decreases the frequency goes up. It is a careful balancing act that is required for the power companies to keep it within spec.

If it was allowed to drift more the power companies wouldn't be required to spin up more capacity that then goes unused.

Creating a pure enough crystal and cutting it in the right format was once very expensive.

There was a time when all the clocks relied on mechanical timers, and a MW sized electrical generator is more stable than anything you can put at your living room.