look at Western architecture. over the course of the last century, our most expensive constructions have gone from beautiful detailed brick buildings to (in my opinion) awful uniform brutalist and modernist slabs of concrete and glass. white walls and big windows

because of a wave of extremely poor taste in the post-war architectural community, many of the skills required to make those older style buildings are gone or no longer common. it's easy to see something like this happening in ancient Egypt. perhaps the Nile delta didn't fully flood for a few years, or there was a bad war. or it just became too expensive to use the old techniques and they died out