Why is Boo Radley an outcast in To Kill a Mockingbird?

He is an Outcast because he is so despised by his crazy parents early in his life and then becomes the feared recluse of the neighborhood (though he is only an innocent like the figurative mockingbird).

Finally, and most importantly, the antagonists, the poor white racists are outcasts to the readers who inevitably reject them and their awful values. That is the author's purpose, to have readers reject the powerful perpetrators of evil, to turn the tables on them and have goodness prevail in the end.