Did you even read her article? She gives a very clear reason.
Walmart sells 140k skus, sorted on shelves and available in small quantities, and has lots of cashiers so you can get in and out reasonably quickly. Costco sells 4k skus, mostly in bulk, and forces you to wait 20 minutes to leave the store.
You know what? You're absolutely right. I went down a little wormhole of McArdle-vs-Other-People link following, watched 12(!) minutes of her and Dean Baker going at it on video about whether the fed can reasonably call asset bubbles, and lost my place; I had a different McArdle post in my other window when I wrote this.
The irony is, I was particularly poorly served by the advice I offered here, since skipping straight to Yglesias totally screwed me up.
That's clearly one of the reasons she gives, but I wouldn't call it a clear reason. It doesn't make the point she cares about, as she herself says. Her claim (approximately) is "Costco's way may work for Costco but it wouldn't work here, because numbers and business". But people used to argue the same about how it would never work for Costco either.
Walmart sells 140k skus, sorted on shelves and available in small quantities, and has lots of cashiers so you can get in and out reasonably quickly. Costco sells 4k skus, mostly in bulk, and forces you to wait 20 minutes to leave the store.