Usually domestic sales in small bags are a tiny fraction of the amount sold in large bags to commercial users. When consumption switched to home use then those small bags sold out quickly. The small bags are still there in the supply chain, it's just that the demand has gone through the roof so it's hard to find them.

When the cheaper 20lb bags sell out, people buy the 2lb bags.

My guess is that more production is for the 20lb bags, and the 2lb bags usually sat on the shelf longer.