1. Windows 11 now ships with quite a decent and powerful image viewer/editor that covers most average users' use cases, therefore lowering the demand from people to go out of their way to find alternatives, like in the Windows XP days, which is a good thing (less likely to go download malware from the first Google result of "image viewer for Windows XP").
2. PC usage behavior has changed a lot since then. Many people don't even have PCs at home anymore, and people now have most of their pics in the cloud or on their phone or some external NAS that comes with it's own browser viewer app, instead of hoarding them all on their home PC hard drive, further lowering the need to seek out dedicated image viewers to manage giant offline collections of digital camera pics(I mean I still do, but I'm a minority nowadays).
These two factors combined meant the death of the third party PC image viewer app. Yeah, Irfan might be "the best", but the need for the best in this sector has declined significantly, and most users are now fine with "good enough".
And yes, at times it flickers and or images can tear.
I mean why would I? If all I need is viewing a couple of photos every now and then, cropping and rotating one or two and drawing some circles on them to highlight something in a screenshot and Windows already does that then why bother with Irfan other than habit and nostalgia.