What are some very good reasons an up and coming fighter's career is shortened?

Im not talking about the obvious champions but the ones that couldn't stand the test.To be a good fighter or boxer doesn't consists of not getting hit.You can be competitive or undefeated and still be punished badly.Gatti was an example. But at the very least a decent defense.To hit or not to be hit...That is the question.I ask because there have been so many good fighters that never really got off the ground.I wonder where they fade away and go.After trying to become contenders.Do they get regular jobs if possible or work somewhere in the fight game.Too much punishment would be one guess.what are some others? And perhaps its why Mayweathers defense is so important to his success. Not real sure i said that second line correct but you get the idea.Thanks.