Because there is no perceived shortage of 12ga. If someone more important than I said "there will be a shortage of shotgun shells", then people would go out and buy shotgun shells - creating a shortage.
Just FYI: that quote is her "signature line". It shows up at the bottom of every post she makes. It wasn't necessarily directed at you.
Also, welcome to INGO.
9mm PPU FMJ in stock at Sportsman's Guide
50 Rounds 9mm Luger Fmj 115 Grain - 954660, .357 Sig Ammo at Sportsman's Guide
$13.10-13.79 a box + shipping NO LIMIT