I don't think there can be a physical reason since x570 is current and I think all the Ryzen generations work on it. It has to be some feature that x570 has that the others don't, maybe the new chips require pci-e 4.0.
All the B450/X470 owners must be pissed off they won't get 4000 chip support. That's Intel levels of dick-move upgrade your motherboard every two years garbage.
I'm miffed, but really only because I don't want to have to get another monoblock/have to redo my whole loop (I put some slack in my tubes so I could unmount the monoblock and swap CPUs), though I guess I could just upgrade everything (mobo, CPU, GPU) when HDMI 2.1 cards come out and sell the PC whole/shift it over to HTPC purposes.