Because violent revolts elevate violent leaders. Because violence is the last, worst option for influencing the behavior of your fellow humans. Nonviolence isn’t more effective than violent political action if all you want to do is swap out who’s in change, but it is more effective (I would argue necessary) if what you want is a nonviolent society governed by a nonviolent democratic government. Once both sides have devolved into violence, really the only thing that sets policy is which faction is able to inflict the most pain. It also proves the fascist rule of “everyone is ultimately violent, so your best bet is to stick with the violent team that shares your religion / skin color / flag / etc.” and dominate through might, rather than trying to build a genuinely peaceful coalition that could, if empowered, build a genuinely peaceful government that makes its citizens’ lives better.

Or, to put it another way, you can use The One Ring to defeat Sauron, and you may succeed in defeating him, but you will corrupt yourself in the process and become the very thing you sought to destroy.