First Elephant in the room - Some people are simply not capable of being in the the LHS. Plain and simple. Doesn't matter how much training you throw at them.
Second Elephant in the room - Not all airlines have the same standards, and yes, they do vary from outfit to outfit.
I don't think it should be a "Right of passage" to the left hand seat. I have seen many an FO, who just plonked themselves in the right hand seat for donkeys years, with no effort or commitment to study or have a decision making model, or even try to make a decision, then expect to pass a command course.