"Popular" inexpensive calibers are in " short" supply because they are used more than the more expensive ammo. Most of the time Mr. Shooter that buys ammo to just go to the range and pop off a few isn't going to buy premium ammo when he can find some for about 1/2 the price. In my area 22 lr is a feast/famine type of supply. One day the shelves will be running over and a few days they have been sold off and the lightly filled shelves are waiting for the next shipment (next month?).
As for 44 Special and 45 Colt, neither are a money maker, high traffic caliber and not as popular (that that I have seen are high priced even for the plain, LRN mild loads). In my town there are only two places to buy factory ammo and they won't stock something that doesn't sell. While I really like 44 Special, I haven't found any factory ammo for several years and that was at the indoor range I used to frequent in LA...