At least where I grew up we didn't call this bullying. We called it teasing or just being a bitch. Now that teasing is considered the equivalent of physical violence it is treated more seriously, but teasing is not the same as bullying. Getting someone to obey you via physical violence, even torture, is just not the same kind of thing at all as saying mean things to someone. It just isn't.
There is a world of difference between teasing or being a bitch and harassment. Throwing the occasional insult is not harassment (at least in the legal sense of the word). Harassment can be far more damaging and harder to repair then physical violence. A black eye will heal in a week or so, a month. If someone decides to convince everyone that you like to fondle little boys the damage can follow you for the rest of your life regardless of the validity.
One of the many differences between sticks and stones and words is that words don't cause any physical harm, any physical damage. In fact they don't have to cause any damage at all if you just ignore them. You can't ignore someone hitting you in the head with a rock or beating you in the face with a stick.
It depends on the severity of the harassment or violence. While you would have a hard time ignoring someone using a stick or a rock on your head, you can ignore someone pushing or shoving you. And while you could ignore someone insulting you it's very hard to ignore when someone convinces everyone you know that you like to fondle little boys.
Those two girls are not responsible for that other girls death. Yes they contributed to it in the same way that a pretty girl rejecting an ugly guy can lead to his suicide, but they are not directly responsible for her death and cannot be held responsible for it. No one forced that girl to jump. It was a decision she made for herself and I suspect that the teasing was only one factor in that decision.
No one is being charged for her murder. The teens are being charged for what they did, aggravated stalking something that goes far and above throwing a view insults at the victim.