Like a lot of things in a fantasy world, you don't need a great, perfectly plausible, rationalized explanation for everything that fits with everything else, at least not with most players. You do need some semi-plausible, "if I don't think about it too hard it works," reason for anything serious, at least with most players I've encountered.
I think it's because interested players want to find the pattern, and for that, there has to be a pattern there to find. For example, there's the old favorite, where the ruins 1 day's travel north of here haven't been investigated recently, because it was supposedly cleaned out years ago, and the nearby civilization has had other things keeping them busy. But recently, something new moved in. Or something new opened up. Or awakened that was never found. Doesn't really matter, and doesn't stand up to strict scrutiny, and that's OK. That of course, presumes that the local civilization isn't a major city.
That kind of reason is enough for a player to make some deductions, once they learn the nature of why things have changed. Learning the reason why there is something going on in those ruins now, might change how the party chooses to approach it.
If you want something more systematic, there are ways. I did one campaign where a rather malevolent set of deities were going out of their way to introduce minor catastrophes any time civilization started to settled down or expand. Not enough to knock it back to immediate apocalyptic effects, but enough to change that timeline Chris discussed from working out. Civilization was still recovering, but it was more 10 steps forward 9 steps back instead of the more usual 2/1 ratio. There's still an end state, but you can still plausibly keep that going for centuries, or even millennia if you stretch it. Picture Western Europe needing 5 or 6 centuries to go make the same recovery from the Black Death that was done in 1 century.