It wasn't cheaper for any physical reason. It was cheaper because we regulated it that way on purpose.

We've intentionally made it unconscionably expensive to bring anything not built to current standard back into service even in a limited capacity (e.g. sublet a factory into smaller space) because we have because this stuff is mostly the purview of local governments who seem to optimize for some middle-ish ground path of "what makes Karen screech least" and "what makes the professional developers who know everyone in government happiest". There's various exemptions for small residential stuff, but at scale it's all just crap that tends toward "don't allow anything that isn't a new build or a high dollar revitalization project"

Seriously, go to your local zoning board, planning board, etc public facing meetings sometime. The shit they put people who just want to spend huge sums of money to develop stuff, run businesses etc, in your city/town through is beyond the pale. And then some "professional" shows up with a BigCo packet about "here's why our toxic waste dump on the ground floor with a strip club on the top floor can go beside the school" and they can't approve it fast enough. You'll be looking for bulldozers on facebook marketplace before the meeting is half over.