One reason I think she titled her book the outsiders because the greasers are considered outsiders from their society. The greasers never really accepted who they are and the socials grow up adored by the city. Since the greasers are outside of the social class the popular kids beat up on them because no one sticks up for them.
I think that Ms. Hinton is referring to Ponyboy and Johnny when she titled her book Outsiders