Originally, everyone wore a tallis even before being married. During the black plague, when people were dying at an extremely high rate, they wanted to encourage people to get married earlier, and have children, so that the communities would survive. To incentivize it, the Rabbis at the time changed the rule to be that you wear a tallis after being married. People married earlier to wear the tallis. (At least, this was what I was told by a Rav a number of years ago.)
I would assume that the groups which wear a tallis before marriage never had the rule changed, so they still follow the original customs.