Technical. Think about it a moment: a day repeats, sun up, sun down. Therefore, anyone designing a clock would think of something that represents day as repeating, as well. The easiest way to make that is with a rotating wheel with a mark on it. It didn't have to be actual rotation, either: a sundial doesn't actually turn, but the gnomon casts a shadow that rotates around the dial. There were linear solutions, like the water clock or clepsydra, or the hour glass- but they had to be re-started, re-filled. Gear trains could make subsets of rotations, so that one shaft could turn at 1/60 the speed of another- and again, that was the easiest way to make something repeat over the course of a day. So, for almost all clocks, there was a dial, a round thing with a hand or hands showing a repetition of time. This happened quite early, as can be seen in the Antikythera Device of c. 100 BC

Now, once that dial is there, there's a whole question as to what shape the clock case would be. For efficient use of space, pocket watches tended to be round- no bigger diameter than the dial so they could fit in a pocket. But for small house clocks to big town clocks, efficient use of space was not necessarily a big priority: ease of design and construction, ease of access for repair, oiling, cleaning etc were. So, plenty of these- tall case clocks, mantel clocks, wag-on-the-wall clocks- had cases that were anything but round. Working with wood or metal, it's easier to make a box that's square or rectangular than one that's a cylinder or sphere, and so many clocks of the 18th and early 19th centuries had cases produced in regular cabinet shops, using dials and movements produced in other shops or factories that specialized in making them.

A very good, delightfully-written book on the development of time -keeping devices as well as changing attitudes towards time and measuring it is David Landes' classic Revolution in Time