Why do they bully?

I have been at the same school sense I was little but I never really ever had any problems until year 5. I had these two girls who were new. They hated me because when I was nine my dad died and they said I talked about him too much. That year I also lost the only friend I ever really had. She decided that those two girls were better and cooler than me. That’s when my depression really started and I really wasn’t really happy much that year. I always thought it was my fault so I pretty much quit talking and that stuck until year 8(which I’m in now). I found someone who would actually talk and play with me year 7 and she has been there ever since. I have six girls in my class and we all have been there for each other since we were little but I feel like I am still left out of things and they always add one more person whenever we are doing something and leave me out. So I also walk fast and I got a nickname and I get called it everyday. I try to ignore it but it hasn’t changed hearing it. I have been called names and so many other things . I just wish there was a way to escape it without harming myself and leaving my school. I still have depression and I don’t think that will ever change.