I assume the producers feel that they need to have an older, more provincial and somewhat unacomplished woman on the show as someone the middle american female television watchers can identify with.
They probably think that not everyone identifies with the ambitious gays who are currently in the lead in the competition and they probably feel they need to keep Marta on for a while at least to keep the interest of a certain demographic. They had an old self taught with poor taste on until the very end last season.