Convienent store=/= grocery store
They have different purposes. You're not going to buy items in bulk,vegetables,cooking supplies, everyday items at 7/11 since they don't even have that, and you won't see me hanging out with friends to get a soda or coffee that Walmart. I'm also not going to be at a grocery store daily compared to a convenience store. The 2 aren't really comparable.
Location of the 2 places matter too. In my town(and for what I've seen) 7/11's are located near residential districts, Main Street, popular hangouts, etc because people really only go there if they pass why one. Walmarts are usually located far off with other mass stores because they need more space and people need to go there. So of course people are going to go to a connivence store more often.
Both suck too.