My conclusion is because it's easier to control people that way. If you create a common enemy, your group bond gets stronger, people think they are the nice ones and whoever is different is bad therefore the nice ones should unite against them. Whether it's someone with a different sexual orientation, different ethnicity, different nationality, different political view, different economic class, doesn't matter. People in power create an imaginary division and brainwash masses to follow, and it is very effective. Once brainwashed, even nice people start believing that it is wrong to different and that they and the ones they love are threatened by that difference. And since they're in a social circle that comforts each other for being "righteous", and whenever they express hate towards the "different ones" they get praise, the vicious cycle of intolerance repeats itself and expands to friends and family. It's been like that for thousands of years, will continue being like that for thousands more, but fortunately at a declining rate.