From my experience talking with all of the manufactures, here is the repeated reason~
The reason why ammunition manufacturers can increase capacity on center-fire cartridges is because that machinery is fairly inexpensive (compared to rimfire machinery) in the big scheme of things. A guy with a couple of Dillons in his garage and some time on his hands can be a center-fire ammo maker if he can get components—especially primers. And, the big guys can make center-fire primers or buy them from other makers more easily than one can prime rimfire brass.
Expanding rimfire production, not so much. A rimfire ammunition plant requires a priming area that is something that has not been newly fabricated in the United States probably for 40 years. Of course, the big American ammunition makers have updated theirs, but they have not added any brand-new facilities at new locations (Winchester did move its rimfire plant from Illinois to Mississippi). It is the priming operation of rimfire ammunition manufacture that takes the large amount of production time.
Frankly, it’s not easy and there are numerous safeguards in place because this is a fairly dangerous manufacturing operation trying to squeegee the wet priming compound into the case rims of rimfire cartridges. And the manufacture of priming compound, which is highly explosive, is not for the careless or squeamish. And I am also unsure what trying to obtain financing and insurance for the creation of a new rimfire plant would be like. And if billions of dollars were to be sunk into a new rimfire plant—if a location could be found and approved—would the demand stay high enough to justify it?
That blurb was taken from here if you would like some more detail.
I must say that the longer the shortage goes on, the less I believe this explanation.