The Roman west was overrun by barbarians, ran out of free land and slaves, and collapsed. Re-creating a civilized order took a few centuries. When people don’t have to spend their time fighting for food and hearth, they can think a bit about medicine and engineering. When people have the leisure to think, things steadily improve. Natural disasters (like plagues) are always unexpected and take away the time to think. (Plus, you can’t think when you’re dead.)
There was no “Dark Ages”, as invented by 19th century haters of the church. They made it up as a rhetorical tactic. There was steady improvement, and it’s just that improvement is not linear, it’s exponential. The curve of improvement not only slopes up (hence the word improve) but it has a naturally increasing slope. The slope just reached a point where all of mankind began to feel it, and we call the “Industrial revolution”. Then, other “revolutions”, all of which are just points on the line where some opinion leaders notice the steady march of improvement and create an interesting phrase.
The Dark Ages term, if I recall correctly, was coined by historians who were referring to a time period where there was a dearth of written material created.
(Plus, you can’t think when you’re dead.)
You think?