Not sure if it's true that mean people fail. But here's a theory as to why nice people succeed: niceness is correlated with a sense of empathy, empathy is the ability to understand people unlike one's self, the understanding of people is critical to generating large swathes of change in the world (which is made up of people).

Or to put another way, a smart person who wants to effect change should try to develop a sense understanding of those they want to effect, it's harder to be mean to people whose "shoes you've walked in".