why is it frightening to the community leaders for people to be able to choose their own mate or their own career

The community leaders desire control over the community, and they believe that "sameness" promotes this control. Choosing mates, raising children who are given into your care, and allowing the committee to dictate your career leaves little room for dissention. You can't be unhappy with their choices, and yet, you don't know the difference because you've never been allowed to be happy. The community simply believes what they're told.