Older housing, hot summers, and hard water.

Hard water is the local factor that matters most. Mineral deposits build up around the drain and the overflow, and once the glaze has worn thin the deposits bond into a surface that's porous rather than sitting on top of it. At that stage no descaler is going to fix it, because the problem is the surface, not the scale.

The housing itself runs from twenties bungalows near the older center through post-war tracts, so both cast iron and porcelain over steel are common.

There's also a large rental and turnover market here, and the case for refinishing between tenants is the same as it is anywhere: no permit, no dumpster, no plumber, and a bathroom that can be shown again.