From Brownells:
The biggest issue is, unlike centerfire ammo, rimfire is produced on dedicated machinery that can only load rimfire. Centerfire machines can load many different cartridges, so if there’s a big shortage of .308 Winchester, the ammo factory can switch a bunch of machines over to it and ramp up supply much faster. That’s why calibers that were in short supply in 2013 and 2014, like 9mm or .223 or .45 ACP, came back a lot faster.
Rimfire is a whole other ball of wax because of the way the cases are primed. It’s really, really expensive and takes a long time to set up new rimfire production machinery – so costly that manufacturers can’t spend the time and money to do it unless they’re absolutely sure higher demand is here to stay, so the .22 LR production capacity in the U.S. isn’t going to expand much any time soon. As long as demand for less-expensive rimfire ammo remains so strong, it will continue to be hard to get. Our guys tell me supply will probably stay very tight right through 2016. Wish I had better news – as soon as I do, you’ll be the first to know!