It's because of efficiency. The entire floor becomes a radiant source - this is a massive thermal source. A heat-pump is only part of the equation. You need to

It's because of efficiency. The entire floor becomes a radiant source - this is a massive thermal source. A heat-pump is only part of the equation. You need to deliver the heat still. A radiator is generally a bad way to deliver heat from a heat pump. Radiated floors are much better. About 40% more efficient than radiators from the same heat pump. Not to mention radiators are sort of lousy ways to heat a room anyways. They take up space and leave one side too hot and the other too cold.