Quitting is never easy -- especially for the alpha types who make up the $3 trillion hedge fund industry. Yet some of the best-known founders have thrown in the towel this year after fortune-making decades in the business. These traders all had their own reasons to hang it up. But there were some common factors behind the decisions: the challenge of competing with an ever-rising stock market, the difficulty of raising enough assets and, for many, the march of time.