Another possible explanation are the heavy fueds over resources. Say your fantasy world had four major sentient races, each fighting for metal, and fuel, and the like. It would be scarce since everyone would want it, and imagine if there were green skinned humans with overly large eyes, they would probably become something similair to the blacks in the time of slavery. There are alot of things that can contribute to a stagnant development of technology, including some type of worm that would eat Fossil Fuels, thus giving the planet a large life span and stagnating development. My take, may not make sense to other people, but whatever.