Well, Snow was cast out of the castle pretty fast by Regina after her father died. Although the new sovereign is normally declared right after the old dies, the coronation normally takes place a while later (sometimes weeks). That could be the reason Snow wasn’t crowned and officially a queen, yet.
Plus, Snow is on the run because she’s a fugitive. Regina probably made it that since Snow is a fugitive, she forfeits her right to the throne and therefore, it goes to her instead.
Good point too. If and when Snow gets an opportunity she’ll have a better claim to the throne than Regina to remain.