Solar water heaters are expensive, complicated, fragile, and generally immobile.

Solar panels and heating elements are cheap, simple and easily replaceable.

A solar water heater is just a black box with a tube running through it. Many houses here have one, because they're much cheaper than PV panels to generate similar kW heat. But a sibling comment pointed out they want to go up to 900 °K, you can't do that with those, of course.