Why do people do this?

So one of my ex-bestfriends, that I grew up with (litterly grew up with) got new friends and left me behind. These new friends of hers aren't good friends. They are really mean and rude. But they are also considered "popular". So am I, but not like them, I'm popular because I'm nice. But also she always used to talk behind my back but I let it slide. I have lots of nice friends but she ahs always been a problem. i am no longer friends with her, but she does not know how I feel. But why do people do things like this?