Because they have an added stigma in the world of shidduchim. People pay lip service to the concepts of t’shuva or rising above your upbringing. You hear heartwarming stories of it but as soon as a shidduch is suggested to a parent with a “normal” child, the parents get all gun shy and say it’s “not shyiach”
It’s a wonderful caste system we have set up. And as we know, everything we do has been done since Har Sinai so dont bother trying to suggest a change to it.