I'm sure there were many contributing factors, but my own theory stems from a fun fact I learned in my History of Ancient and Medieval Science course: the idea of the crank, or continuously applying force in a circular direction to do work, in a way that didn't have a natural limit (such as reaching the end of a screw), actually wasn't developed until sometime in the vicinity of the late Dark Ages. Archaeologists have found devices for e.g., grinding grain that are astonishingly close to using a crank motion, but were in fact pivoted back and forth, a vastly inferior solution.

Without the development of the crank, the idea of spinning wheels with pedals or gears would not occur to anyone.