Because too many have proven incapable of maintaining their distance - stores are trying to no be part of the problem as we see if a couple weeks of being extra cautious can actually achieve results.
I understand the reason but it creates new problems such as crowds in a parking lot. An alternative would be for WMart to require people wear face coverings. All joking aside selling tickets(in advance) with an admission time and limited shopping duration might not be a bad idea.