What they had in common is they understood the manner by which thoughts possess people and determine their actions,and not the other way around.
Or the common sense to avoid a battle which would inevitably end in the utter annihilation or continued enslavement of the very people they said to be saving. It was the realization that however barbarous their repressive governments, there lay some kernal of civilization within their oppressors - and they exploited it.
African-Americans in the United States lacked the firepower to carryout an armed rebellion and the education to carry it on through the political process. Although Martin Luther King, Jr. and Gandhi both, firmly believed and carried out "civil disobediance", neither Gandhi or MLK, Jr. would have succeeded in an armed revolt. The Indians lacked the proper firepower as well - after the First War of Independence in 1857, every available resource for violent rebellion was removed. And the African-Americans never had it to begin with.
I would say that their success lay partly in their setting (though vastly overshadowed by their passion and their talent).