reason, (in spite of conspiracies speculated on the internet), is technical rather than marketing (or otherwise conspiratorial).
Even though Windows 10 proved remarkably stable, the few crashes were almost always driver related, Microsoft is now attempting an elimination/reduction of those issues too.
Microsoft's "Mode Based Execution Control" (MBEC) is an important performance feature allowing better isolation of kernel memory, it's included in Ryzen 2 and Intel gen 7, (Ryzen 1, Gen 6 don't have it)
Users wondered why some Ryzen CPUs aren't certified for 11 now you know the answer.
After that, some processors adapt strict driver implementation and prove it, some either don’t or haven’t proven it
This specification details the AMD processors that can be used with Windows 11 customer systems that include Windows products, including custom images.
Then there are other processors that didn’t implement best security design, giving rise to the Meltdown and Spectre Flaws
Even though those CPUs were patched, I don’t think they’ll get certified even if they follow the driver stability principles, but possibly in the future, depending on Intel successfully pitching their case and or adding firmware to the chipset I suppose.
Processors must follow the Windows Hardware Compatibility Program
and drivers running on the CPU also must use DCH Design Principles and Best Practices - Windows drivers
Once a CPU model completes requirements, there might be a process getting them certified.
CPU and 11 compatibilities aren't some random date parameter, nor "so you buy new PC's
If a CPU isn’t certified, it won’t “officially” run 11, but there are work arounds.
Microsoft is working with both AMD and Intel to iron out the floors in their processors so there's a chance some that aren't certified will eventually become.