AMD ran into significant compatibility issues with AM4, with some motherboards not being able to supply the amount of power needed by the newer CPUs and PCI-E Gen 4 support being removed in the final BIOS release due to reliability issues. A lot of motherboards also didn't have enough flash space to contain the firmware needed for the whole CPU range, so the update to support the newer gen had to remove support for the oldest gen.
Turns out it's really hard to guarantee compatibility with several years of third-party products which weren't designed with your new fancy features in mind.
Newer doesn't usually mean more power, though. You can just have a power limit, it's fine. And I don't expect PCIe to get faster with a CPU upgrade anyway.
The flash space was pretty forseeable and they dealt with it, more of an excuse than anything.
Turns out it's really hard to guarantee compatibility with several years of third-party products which weren't designed with your new fancy features in mind.