"Why there is an ammo shortage?"

Because of me!

Just kidding but it is because of all of us. My local big box retailers have supplies of big name ammo in hard to get calibers. Thing is it's all $1/rd or more and supplies are low at that price. I can't bring myself to pay $25 for 20 rds of Federal Hydra-Shok even if it is P40HS2 for my G22. I have stocked up on 180gr ball though. Boxes of 50 are still $18 around here.