Because if you fail after making a half-effort, at some level you can comfort yourself with the knowledge that you didn't fail at something while giving it your all - after all, if you self-sabotage a little, there's your excuse for failure, which might be embarrassing but probably not so ego-crushing as knowing that you gave it your absolute all and failed anyway.
I don't think it's really a fear of success so much as a fear of putting yourself as much at-risk as you may feel you need to be, in order to achieve that success. Plus, major success in a lot of fields - maybe in any field - means a paradigm shift in your life, for better *and* worse. People tend to gravitate to the familiar; if mediocrity is where one finds one's safe, familiar balance, then pushing past it to unfamiliar territory may seem like a scary venture into the unknown.
I don't think it's really a fear of success so much as a fear of putting yourself as much at-risk as you may feel you need to be, in order to achieve that success. Plus, major success in a lot of fields - maybe in any field - means a paradigm shift in your life, for better *and* worse. People tend to gravitate to the familiar; if mediocrity is where one finds one's safe, familiar balance, then pushing past it to unfamiliar territory may seem like a scary venture into the unknown.