The best I can tell, there are a lot of difficult/complicated things that go into a waste-heat water heater: heat-transfer, water-line hookups, connectivity -- it's possible that it's really just a bridge too far for any kind of widespread adoption. The impression I got going through all this is that a dumb water heater fits the bill for most people, and there isn't a lot of demand for something that saves a little bit of money in monthly bills at the cost of a whole lot of added complexity (and up front cost).

planned obsolescence