I think it's mostly because of supply and demand. Ammunition suppliers are producing as much as they can, but going from selling average amounts of ammo to absurdly large orders is a hard thing to do.
And since Mr Obama was elected, gun sales and ammo sales have spiked and plateaued at unheard of levels, and show no signs of dwindling.
I stopped by the one in Noblestucky on my way home the other day, and noticed they had just two boxes of Federal .40 on the shelve so I imposed my own limit and bought both. Tossed them in the trunk for November Deer season in Michigan.