Well lets look at a few things first.
There are 4 sects in Islam. Many on here claim that all Islamic people want to do is fight and kill each other as well as everyone else. Which is true.
But lets look at the differenlty denominations of Christianity. There's about 10 different "main" denominations. Each one of those denominations are further split into different styles of that particular denomination. So in the end there is a rather large list of denominations and styles of Christianity. Why are Christianians so divided?
Judism is also divided into about 5 different denominations.
The answer is that the history of man has been a violent bloody one. Jealousy, greed, envy, anger and other basic emotions have led us into the grand civilizations we are today. A gathering of people all sitting on stockpiles of weapons that have the capability of erasing our entire race out of existance. All because after thousands and thousands of years, we can't control our basic emotions.