It is a lot of reasons.

But in vaccine research one of the first questions you ask is, does the disease itself confer life long immunity? If it does, there is a decent chance to develop a vaccine that also protects for a lifetime but doesn't cause disease.

But the reasons for coronaviruses and influenza not generating sterilizing immunity are probably not the same. Just like the reasons polio and smallpox being able to create lifelong immunity are different as well.

I can only speak at length about natural and vaccine derived immunity from a few bacterial diseases. And those reasons are very different from the reasons for viruses.