In a word, death.
Roughly, because old people die and young people take over. Changing hearts and minds is very difficult, but replacing them is only natural. In the US old attitudes towards race and sexuality have changed to a large degree not because old racists and homophobes changed, but because their more open-minded children took over. In Sri Lanka and all over, this is just the natural way of change.
One potentially very important mechanism of institutional change is cohort replacement. By that I mean the replacement of old guards of organizational members and leaders with newer cohorts who have different beliefs, opinions, and values. It’s strange, when you think about it, that institutional theorists haven’t considered in any serious way how cohort replacement affects organizational practices and policies, even though opinion research indicates that cohort differences explain significant variation in beliefs and attitudes. Cohort differences may often matter more than life stage differences in explaining political opinions and attitudes. Take the case of liberalizing beliefs about same sex marriage. One study indicates that about half of the growth in support for same sex marriage is the result of cohort replacement. Younger generations are simply more open to this practice than preceding generations. We can expect that in a couple of generations, same sex marriage will be legal everywhere due to cohort replacement. (orgtheory.net)
A quote widely attributed to Max Planck sums this up well
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it. (Wikiquote)
Also, Steve Jobs
No one wants to die. Even people who want to go to heaven don’t want to die to get there. And yet death is the destination we all share. No one has ever escaped it. And that is as it should be, because Death is very likely the single best invention of Life. It is Life’s change agent. It clears out the old to make way for the new. Right now the new is you, but someday not too long from now, you will gradually become the old and be cleared away. Sorry to be so dramatic, but it is quite true. (Stay Hungry, Stay Foolish)
In terms of change, I think death and migration are the most powerful and most ignored change agents out there. You could say that war is an attempt to accelerate these natural processes as much as anything. Kill or deport people opposed to you and move on. This is what happened in America and Australia’s colonization, but it is of course horrendous, especially because there are better options available. War, especially modern, inconclusive wars can also delay change because they have a nihilistic, self-perpetuating logic of their own, backed by heavy emotion that does transfer across generations.
However, simple natural death – peaceful and mourned – can do wonders for reality.