See now this has always confused me too...plenty of stores that are legal to be open 7 days a week close on weekends or close early on Sundays or whatever to give people a bit of a break. Why do they oppose being able to run their business as they see fit?
It is not that liquor store owners want Sundays off. The reason they oppose lifting the prohibition is that they realize more than half of the population does their weekly grocery shopping on Sunday. If they are able to stock up on booze at the same time, those shoppers are less likely to visit a liquor store during the rest of the week.