Why do the other kids in Welch pick on Jeannette and her siblings? Was it something about them, or just the fact they were outsiders?

Children often pick on other children that they deem to be different or vulnerable in some way. Jeannette and her siblings are poor, even poverty stricken. They wear old clothes and have a reputation for being "different". Unfortunately, that gives other kids more than enough impetus to stereotype and bully them.