Because it leads to bills like this, and you can't change natural behavior. Kids are going to bully other kids, it doesn't matter what you do, it's part of their natural and social behavior as animals, and no good ever comes from trying to dwarf and drown any animal's natural and social behavior.
You tell a kid to stop jumping on the bed, turn around walk out the room, he jumps on the bed. They are kids, they are going to bully, it's part of who they are at this age and it is part of their social growth. What needs to changes is parents babying them and treating them like little angels, and teaching them to stand up for themselves instead of running to the school crying.
Parents are at fault for not raising their kids strong enough to stand up for themselves and deal with it, not the kids who are naturally, instinctively, and socially growing via an act of bullying that has taken place naturally for thousands of years. It's the new wave of American "pussificaton" causing this issue, not kids doing what kids do.