You generally still need electricity for radiant heating - there's a pump moving water through the radiators.

But I think it's pretty easy to understand why it's unpopular. First, AC is popular in the US, so a system that can reuse the same ducts is a lot less expensive than something entirely separate. Secondly, it's a lot simpler and cheaper to service. If your hydronic heating system freezes or develops an airlock or springs a leak, you might be looking at five-figure repair costs.