They lack the maturity to take responsibility for their mistakes in their past relationships.
Make sure to probe a new person to find out why past long-term relationships did not work and ended.
But when those same "in love" people take off their rose colored glasses, they realize the person they thought was Mr. ("He's so good-looking," or "What a gorgeous woman! If a person is serious about finding an emotionally available person for a committed partnership, there are whole categories of people who should be avoided: people living in another state, those who are still married or in love with someone else, and people with addictions - be it workaholics or drug addicts.
The "booby prize" in life is trying to understand or change the behavior of an emotionally unavailable person.
Perhaps, they are recently divorced or widowed, and legitimately not ready to get involved in an intimate relationship.