Amm.. That means British Monarch in India had conscience, when Mahathma Gandhi was promoting non-violence and hunger strike. Which I don't think is true.

In India it worked because British was sure that if they use force against someone promoting non-violence the retaliation will be uprising of a violent mob. Atrocities in India reached a high were majority of the people were affected by it and fed up by it. Like the modern times it was not possible to sway the public opinion through media of any kind.

Gandhi was a saint in practice‌, but more than that he was shrewd politician, who identified the weakness of British monarchy in India.

I don't know much about American History. But I believe that America not having a conscience could be one of the reason why the non-violence will not have worked. I am not sure if that is the primary reason. Can someone with more knowledge in American History provide an elaboration on this point? I feel there will be some missing context.

Nonviolence also "works" when there is a violent group that makes change necessary, which your movement can then be used to undercut. Like socialists being used to undercut leftists, or MLK's liberal civil rights being used to undercut the black panthers' marxism.