From a practicality standpoint, a round clockface is easier to create a mechanical drive system for.

You can create a digital mechanical face (see: Flipboard style numerical displays) but they usually require more gears and are more susceptible to wear and tear than the gears of a round clock face.

The simplest designs for mechanical digital displays actually just take 24 hour and 60 minute/second circular displays and hide the other numerals as the clock face spins around. Technically this I suppose counts as both analog and digital?

Example:

As for electronic displays? Nah not much of a reason to use a round display unless again, you have an electric-mechanical drive and want to save on gears and parts.