Why are most 7-eleven stores on the north side of street?

This may be true for where you are observing the locations of the stores. 7-Eleven stores are convenience stores, meaning they try to be convenient place to shop. The location should be convenient to the maximum amount of traffic. So if this is an east-west running road, then there probably is more traffic traveling west in the afternoon, so you only have to make a right turn to get in the store, and a right turn to exit.

Think about what is profitable for the convenience stores: beer, cigarettes, and the $5 gallon of milk. Things somebody might pick up on the way home, not on the way to work.