Hi,
The bottom line is, the unsupported processors does not have a feature called VBS, which is a requirement for Windows 11, even though it supports TPM and secure boot. If you enable VBS aka Memory Integrity from Windows, you would see a reduction in performance, hence Microsoft decided not to put it into the list of compatible processors. You can install manually, but it is not worth it as Windows 10 continues to be supported until 2025.
Also, Windows Security Director explains in detail in this video:
Understanding Windows 11 new security requirements with David Weston - YouTube
https://www.youtube.com/watch?v=j6VqSImsZxU