Question: Why did Karen and Paula leave the show?
Answer: Karen: She had good chemistry with Harry Anderson, but not with the rest of the cast. If so, then perhaps that's why her character was written out so soon. Paula was released for similar reasons.
Answer: Karen suffered from Bell's palsy and was unable to finish the season. She was released from her contract. Paula was likely fired because they didn't want to do an interracial romance on a primetime show in 1985.