Really? I'm in the US, and I have hot water radiant heat.
It is a matter of cost. In most of the US, you need both heating and cooling during the year, so to use hot water heat requires an extra heating plant. Forced air allows you to do the whole job with only one plant.
In the New England region of the US, it doesn't get all that hot during the summer, so it is more common to find hot water heating systems, and just a fan for the summer.
In the southern US, heating in the winter is just a theory, so they use forced air cooling, and a heat pump, or resistive baseboard, to take the chill off at night.
-Chuck