It's a real hard problem for localities to get right. On the one hand, you want to preserve old buildings. On the other hand, buildings are expensive as hell. And when those ancient buildings were constructed, they were built by modest people. They were never meant to be super expensive to maintain. But over the ages, techniques that were commonly known then became rare specialties now. Every Medieval peasant could thatch their own roof. Now you'll pay a fortune to bring in an expert in historical construction techniques to do it.

Yes, it would be ideal to require everything be maintained to historical standards and methods, but at some point, people just stop trying. People only have so much time, money, and resources. If you make it too difficult to renovate properties, they can't be updated for regular use. And if they can't be used, then who will put in the money to maintain them? The properties just end up being abandoned and slowly succumbing to the elements.

Some historical artifacts are easy to preserve. Give me a 300 year old violin and ask me to take care of it, and I can do so for almost no cost to me. Just keep it somewhere safe and dry, and it will be preserved just fine. But buildings require constant maintenance, and they are built assuming the easy availability of certain maintenance techniques. Requiring excessive adherence to historical standards does not result in more preserved homes; it just results in old homes being left to rot.