I don't think honesty comes into it really, I think that cultural paradigm shifts are far harder to instigate than that.
As you say, these social advances snowball. Once you start treating some people more fairly, others will be more inclined to demand rights for themselves. But getting that snowball rolling is not easy. Think where we've come, from Magna Carta, Civil War, the birth of unions and welfare out of the industrial revolution. We knew about other cultures that were less cruel, and had philosophers to give even more just visions of society, but we still thought we were "right".
Let's take democracy as an example, because there is a consensus that it is "the least worst" political system. But then think of an Afghan tribal elder, who has been responsible for the well-being of his people for many years, through conditions most of us could barely imagine. Now imagine telling this man that an 18 year-old girl who's never left the village should have an equal say in political matters. I can't imagine that would go down well, just as when men of certain social standing were given the vote in Britain, the idea of women voting would be largely scoffed at. It is a process, and not one that we know enough of to reliably replicate it elsewhere, especially not in countries who's culture we don't fully understand, and especially not in a mere matter of decades.