From an /r/technology thread, emphasis mine

It isn't really even about the software, it is the hardware requirements. Win 11 with the TPM modules and required chips will always mean it has limited share at least for some time. Many machines simply can't upgrade to Windows 11 from 10. Windows 10 will have staying power like Windows XP due to that.

I like Windows 11 and all machines updated except for one from 2016 that is more of a fun computer that just can't justify putting money in it for the TPM or updated chip.

Yeah, this isn't people downgrading to windows 10, or refusing to update. Rather, this is because people are upgrading to Windows 10, and LOTS of existing computers are old and do not meet the hardware requirements for Windows 11.

Windows 10: 70.03% (+0.96 points) Windows 11: 25.65% (-0.97 points)

Those are percentages of the total. It may be some people downgrading but it's not worth it. More likely there are more Windows computers and the bulk of them end up on windows 10.

The "Microsoft is washed, people don't like 11 anymore" angle is tempting but I think it's the less likely reason compared to an increase in sold hardware that literally cannot run 11