OBAMA!
Actually it's much more complex than that, there are a lot of factors but this article summarizes it well:
http://dailycaller.com/2014/10/02/ok-really-whats-going-on-with-22-rimfire/
Really...the situation is pretty simple.
-First off, the rimfire boom started after Obama was elected in 2008. There was a serious run on ammo (stupidity driven), ammo prices shot up, people wanted .22 alternatives, makers began making more 22 "clone" firearms like the S&W M&P 15-22, people began shooting more and more rimfire. The clones were mostly semi auto and require higher quality ammo to run properly.
-Things were going well until 2012, after the Newtown shooting caused another dumb-driven panic. This time, rimfire became a focus. Unlike centerfire, one could back order hundreds of thousands of round in mere minutes. Look at it this way, one 15,000 rd case of Blazer bulk (a choice ammo for semi-auto rimfire) was $150 in Dec of 2012. One could back-order as much as they liked until the big online retailers put a stop to it about 3 weeks after the panic. It was simple to plop $1500 on your CC and literally back-order 150,000. Sites like Brownells, Midway USA, etc reported having years worth of back-orders made in under 1 month. I remember an article written years ago where Midway stated that if they got normal shipments of rimfire in, they literally had 3 years worth of back-orders to fill.
The "quality" issue is apparent in the fact that when you do see rimfire in stores, is shit like Remington Golden bullets...ammo that is such shit that S&W called it out by name as ammo you should never run through their firearms. I have not seen a single 50rd box of Blazer bulk on a store shelf in pushing 4 years...not one. Online grey market however? Plenty to be found everywhere you look...and 2-4x times retail.
This is completely caused by gun owners...it's literally all their fault for being panic driven, greedy fucks. Like any other panic, hype, or gouging driven market of this new age, I just pull out of it. I have enough stored 22 to teach my daughter to shoot. Once I'm done with what I have, if 22 doesn't hit the market, I'll either mothball or sell the firearms before I pay some neckbeard 4x the asking price for ammo.