IIRC, it was due to spectre/meltdown [0], where a rather obscure design flaw in Intel CPUs of the time made it possible to slowly read the kernel memory space from an arbitrary process.
Microsoft added software mitigations for it to Windows 10, but they came with a price of a ~20% slowdown.
By the time of Windows 11, Intel fixed the issue in the newer processors, so instead of supporting both old CPUs (with slowdown) and new CPUs, Microsoft just scrapped the mitigations altogether and limited Windows 11 to new CPUs only.
Microsoft added software mitigations for it to Windows 10, but they came with a price of a ~20% slowdown.
By the time of Windows 11, Intel fixed the issue in the newer processors, so instead of supporting both old CPUs (with slowdown) and new CPUs, Microsoft just scrapped the mitigations altogether and limited Windows 11 to new CPUs only.
[0] https://en.wikipedia.org/wiki/Meltdown_(security_vulnerabili...