Mindfire
Istar
I hope I didn't miss your point, but I could have.
The initial question was "how do we explain these societies caught in technological stasis for centuries or longer"? My belief is that you need a darned good reason for it.
I still think that technological advance when the resources are available are a standard part of the human condition. I just used three examples that came easily to mind, but one of course can mix and match and tailor to taste. Your group of people who avoid writing things down (for whatever reason) might still want more crops so their children don't starve. The people who are not agriculturalists will still want better knowledge of how the seasons change to better predict where to obtain certain foods, or better tools for hunting or gathering. Pyrokinetics still need a fuel to burn so they might quite like fossil fuels that are lighter and more efficient and burn longer so they don't have to re-start their fire places every ten minutes. They also might want devices to better focus the heat, or to distribute it more evenly, or to make sure the smoke from the fire doesn't leave soot around the inside of their dwellings.
Technology is also a great force multiplier and result enhancer even when you really don't need a "thing" to achieve what you want. A human can get himself some basic stones and crush grains by hand, or he can throw a rock, no serious technology required. But despite that they tend to build water driven mills to do more of the same in a better way. Or he might sharpen that stone he throws or invent spears and bows. A man can see with his own eyes but a telescope or a microscope still allow him to see and learn much more, and that is without even talking about x-rays etc.
And the question of how one justifies a lack of advancement in technology in societies over centuries is not a eurocentric one. Virtually all societies develop technology at different paces and for different purposes. The Aztecs and Egyptians and Zimbabweans built some amazing structures without a wit of help from western europe through their own technological advances.
If a group of people has things they desire, or goals, they will seek ways to get those things more effectively and to achieve those goals. One of the ways of doing that is technology. I think if a society is stagnant in technology in a tale, no matter where set, or with what goals and abilities, you need a pretty strong story rationale for things being in technological stasis. No European bias involved at all. The same analysis to my mind would apply to any group of people no matter where they are in the world, or what special abilities they might have.
On further consideration, we might be talking past each other. The point I was trying to make is that a society can appear to be stagnant and yet not actually be stagnant. Because typically the layman's metric for whether or not a culture is stagnant is "are they becoming more like the modern Western world?" Just because a society doesn't have X technology that we deem important doesn't mean they're stuck, just that they developed in a different direction. And then there's the semantic argument where the word "technology" can be as broad or as narrow as you want it to be, so a society could have technology without having technology...