Protest failure is often blamed on failing to have a unified message. Peaceful protests allow a wide selection of interests to add to the message. As sustained uprising requires a common goal because it requires more solidarity and mutuality to survive conflict. Those are easier to achieve when the message is plain and clear. Part of why the extreme flank is so effective is that it forces then peaceful protests onto a specific topic where they might otherwise just be noise.

Violence is also pretty much the only way to break into the mainstream news cycle.