Nobody has them in stock.
Or, if they do, they are outrageously priced (cheaper than dirt has 30 round Colt aluminum mags for $99)
I saw an PMAG close on eBay for $100. Dead serious.
Backorder is a way to go, especially if you want 6 or 10 or 20 or some decent number that is just too expensive to buy right now. No telling how long the backorder will take, though, and some people worry that a ban (I don't think there will be one...but it's possible) will happen before their backorder is filled