Which itself is derived form the mains frequency of 50 or 60 Hz depending whether you're from the metric or the eagles/freedom land.
So the digital sample rate is related to how fast the power pant spins it's generator turbine.
Sorry for nitpicking, but the huge country I live in uses metric but has 60 Hz for the mains frequency (and our analog TV standard is PAL-M).
https://en.m.wikipedia.org/wiki/PAL-M
> [PAL M] is unique among analogue TV systems in that it combines the 525-line 30 frames-per-second System M with the PAL colour encoding system (using very nearly the NTSC colour subcarrier frequency), unlike all other countries which pair PAL with 625-line systems and NTSC with 525-line systems.
So presumably PAL M runs at 60 Hz just like NTSC, which again comes back to the domestic AC freq
Also based on how incandescent bulbs will appear to visibly flicker at lower frequencies.