Probably for the same reason your employer does. Not saying it's fair to discriminate against people who have paid their debt to society but I wouldn't be crazy about having a rapist or murderer neighbor.
I'm German. Landlords here aren't allowed to check criminal backgrounds as are employers (with the exception of jobs dealing with children and valuable objects).
The idea behind that is that recidivism is best prevented by letting people be normal parts of society (=being able to work and live in peace) once they have served their term.
