I have the same question on a reading assignment for The House on Mango Street. Maybe instead of trying to fit in with others, they don't have to put on a fake personality and therefore they can be who they really are.
Some people consider that slaves are needed, so whole groups of people are humiliated into the chosen slavery and kept there by being made to feel there is no way out.
Other reasons for excluding...
As you may know, Society can't be perfect. That's just the way God made it. Plus some people just can't help looking down their noses at the people with say dirty clothes.
I believe that he gave them encouragement to accept their lot in life. He promised them that if they kept all of the commandments they would be able to bare their sufferings. We all must bear our own...