This book has this title because it accurately captures the feelings of all the major characters in the book. In the story, all of the people that we meet (and especially the main characters) feel like "outsiders."
By this, I mean that they all feel that society is against them in some way. They are dissatisfied with their lives.
You might think that only the Greasers are outsiders because they are the ones who society looks down on the most. But even the Socs feel that society does not understand them and does not give them the approval and support that they need.