I'm not really surprised. While I can only speculate as to AMD's reasoning here, X470 and earlier boards are a mixed bag in terms of PCB quality, VRM implementation and firmware. X570 design guidelines mandated a lot of changes people aren't generally aware of for the sake of PCIe and CPU compatibility going forward. I think AMD has done a fine job delivering broad CPU compatibility and it doesn't bother me that AMD is limiting its Zen3 to X570, B550 and newer chipsets.
The fact is, there are a lot of problems with maintaining broad CPU compatibility for long periods of time. We've seen it time and time again with Socket 7/Super Socket 7, socket 370, socket A, LGA775, LGA 2011v1-v3, and of course, AM2 and AM3 just to name a few.