> You might expect the power supply to be a simple buck converter. However, the power supply uses a more complicated design to provide electrical isolation between the spacecraft and the clock. I'm not sure, though, why isolation was necessary.

A reason very simple, you don't want a short through the clock to set your space, or aircraft on fire!

A big reason why a lot of avionics still works on HF AC power, and not DC is that it makes isolation, and providing arbitrary voltages very easy.