I’m really not entirely sure. I haven’t been around one since I traveled overseas and that’s been a while now. My new place has a new furnace and the old one we never got around to doing improvements so I just remember the old complaints.

Thermal conductivity seems likely. How many feet of pipe can you cram in there, especially if you have to insulate so you don’t catch the building on fire?

Or can you make a 3d counterflow system so you don’t have to insulate at all?

ETA after further reflection, I believe they generally worked better in Europe and that would most likely be down to 220 vs 120, so you may be right about power density.