For whatever reason they weren't building domes anymore in Europe where they had been (the fall of the Western Roman empire sort of changed priorities for a while). And after a generation of not building domes the artisans (in Europe) lost the ability to do it.
There's a much closer example: The US spent the 60s developing the capability to land humans on the moon. And they were successful. They did it about, what, 7 or 8 times? But then they stopped doing it. And now some 60 years later they're having a hard time doing it again (see the woes of NASA's Artemis and Boeing's Starliner). Imagine if the pause wasn't 60 years, but several hundred years.
I only want to add that's not something that we have a hard time doing because it suddenly became hard, but because it came out of fashion.
We had a really good time thanks to cold war and flexing between both sides, but now it's a just a enormous investment with no return. It's really sad to see NASA running on a shoe string budget, and Ingenuity being nothing more than a glorified student project that happened at the right time in the right place.
There's a much closer example: The US spent the 60s developing the capability to land humans on the moon. And they were successful. They did it about, what, 7 or 8 times? But then they stopped doing it. And now some 60 years later they're having a hard time doing it again (see the woes of NASA's Artemis and Boeing's Starliner). Imagine if the pause wasn't 60 years, but several hundred years.