Because it was the cheapest ammunition available at the time and when 500 rounds cost $20 and everyone knew that prices were going to spike after Newtown, people went to Walmart, Dick's, and other lgs and sporting goods stores, they bought bricks of .22 by the shopping cart.
I think that was the
2nd reason on my list of why 22 disappeared.
The first reason (in this perfect storm) is folks like me that didn't own a 22 or had only single shot 22's.
I had no interest in 22 until the glut of nice 22 clones of my bigger guns - AR15's, 1911's, etc. I went from buying / shooting no 22 to shooting a LOT with my various 22 semi-autos.
Just cruise over to YouTube and see how many folks are shooting the various semi-auto 22's that have come out over the last few years - there's apparently a LOT of folks like myself. I know I have a number of friends in the same boat as I, and I've seen how many of those M&P 15-22's have gone out the shop doors over the last few years.
I feel the increase in the amount of 22 being shot has gone way up over the last few years. Couple that with ammo makers not wanting to expand (some finally have), the panic buying, and the shortage bringing out the flippers and you have the makings of a perfect storm AFA 22 shortages.
BTW - I heard quite a while ago Tom Gresham said it took a while, but some 22 ammo companies finally decided when the buying tapers off, the new normal will be much higher than it was in the past, hence their decision to finally expand production.
If the shortage was only caused by panic buying, etc., their would probably not be a "new normal". The vast increase of folks buying / shooting much more ammo due to all those new semi-auto 22's is no doubt what has caused the new normal to be quite a bit higher than in the past.