Most of the stores in the SLM are family owned and operated. I visit at all times and days and almost always there are the exact same people behind the counters. They recognize me, give advice on what's good to buy that day, make small talk, and give me deals or "extras" sometimes -- this is the sort of thing that makes shopping at the SLM more pleasant than a typical grocery store.
If they were forced to open Sundays and Mondays the owners would have to hire minimum wage help to cover those extra hours. Then you'd be served by the same indifferent and unknowledgeable 15-year-olds that staff Dominion and Sobeys and there'd be no point in going to the market at all.