My thinking/opinion…
12ga shells get shot up in high volume, whether clay shooting for fun, competition or waterfowling. 12ga target ammo is almost a lost leader product, with the really good margins being in premium waterfowl ammo.
.410 ammo is consumed in much smaller quantities. Skeet competition is about dead, beat out by sporting clays - which is dominated by 12ga. So not many clay shooters are consuming .410 (and if they are, many are reloading). A box or two of .410 ammo sold to a grouse hunter is barely worth running a production line for… so the ammo manufacturers don’t run much .410 at all.
The problem lies in a dwindling supply of all ammo from the COVID debacle, which saw most folks buying ammo by the case instead of by the box, They couldn’t just buy a couple boxes as needed… and every time they did find it, the price was higher than last time. So, they’d buy a case because ammo has damn near become an investment. And if they couldn’t find that case when they wanted it, they’d buy three when it showed up again, so the ammo shortage became a self fulfilling prophecy. That broke the system entirely. With millions of rounds of 12ga coming off the line, that gap has started to close. But with a slow trickle of .410 being produced, the gap is either not closing, or widening.
No ammo on the shelf, or just a short supply at ridiculous pricing, means the guns that shoot that ammo will be in very low demand.