When the supply pipeline is emptied it takes time for it to refill. The ammo has to be manufactured or imported, shipped to distributors, shipped to wholesalers, finally getting to retail outlets. It's like an empty hose if you turn it on it takes a while for the water to get to the end of the hose.
Even before the ammo is manufactured, just think of the huge increase in demand for raw materials. Inventory is expensive; companies do not like to stock product (or components for products) that may not sell.
Just imagine the effect on the supply chain if all ammo manufacturers simultaneously request 2x or 4x their normal purchases of raw brass, lead, and copper. I'm not saying there isn't something nefarious going on, but ramping up production volumes can take time.