This is because the book largely focusses on a group of teenagers, the 'greasers' who are generally looked down on for their relatively low social status. This is emphasised by their constant running battles with the Socs (short for Socials) who are from wealthier, higher-class backgrounds. The narrator of the book, Ponyboy Curtis, is one of the greasers, and he feels somewhat at odds even with his own group at times.
The title could also be taken as referring to the whole experience of being a teenager, when one is no longer a child and not yet an adult, and therefore something of an 'outsider' in society as a whole.
It's entitiled 'The *Outsiders*' because the main theme of this book focuses on the long-term rivalry between the two social classes - the Socs and the greasers. The greasers see the Socs as rich, spoilt and arrogant people, while the Socs despise the greasers as poor and filthy people.
However, when Ponyboy (a greaser) met Cherry Valance (a Soc) at the drive-in in Chapter 2, Pony realises that there is more to the wealthy Socs than meets the eye, which has made him change his perspective of the Socs. Thus it can be said that the greasers are like 'outsiders' looking in at the life of Socs, thinking that the Socs have it all; while the greasers are unaware of the problems the Socs face 'under the surface'. Similarly, the Socs are like 'outsiders' looking in at greasers and thinking that all greasers are the same.