Bottom line - people are going to bully because it creates esprit de corps...

...and if society says that one group is off-limits, then a new group will be nominated for the role of common people we can feel superior to. 

//Older people will tell you that anti-ginger malice is a strange and new thing in Britain, or that in Italy, for instance, having red hair is considered rather attractive. Perhaps it’s because the prejudices of older generations – racism and sexism – have become unfashionable and strictly penalised that these new ‘prejudices’ have emerged. Teasing gingers is possibly the new means of establishing a pecking order in schools, of establishing group cohesion among bullies. Unfair prejudice is indeed hydra-headed.//