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Just because a person knows the machine, the process, the customer, the material, or the norm does not mean that he knows how to lead people. It's even worse when someone in charge is “good with the boss” but lacks the authority, criteria and ability to hold a shift.
Then the company loses two things at once: a good operative worker and peace of mind in the team.
The leader must not be chosen by sympathy, habit or impression. There has to be a clear criteria and system, to go through the deputy process, a set of training before taking responsibility, and a system that measures how that man leads people, not just how long he's been at the firm.