Why does the tragedy in Christchurch seem to be leading to genuine reflection, and the posibility of real change?

Jacinda  Ardern, to answer the question in the post title.

Other major tragedies have occurred - for instance, Port Arthur, in Australia, and many in the USA.

The Port Arthur mass-murder-by-shooting led to good gun laws in Australia, but we were led by the morally bankrupt, power hungry, and in many political and social areas (I don't know what he is like in his personal and family life: he MIGHT be completely different - genuinely caring) downright evil John Howard. I consider he was genuine in his desire to address what he saw as "the problem" around Port Arthur, but his vision was limited by his addiction to macho stereotypes. Thus, the social problems that had led to this terrible event remained not only unaddressed, but even unacknowledged.

In the USA, the deaths of so many children at the terrible Sandy Hook mass-murder-by-shooting came so close to causing change, but the gun-and-individual-freedom-over-social-rights lobbies have decades of experience at circumventing what is good for the broader social good, and US President Obama was starting to be constrained by his position (I have often considered it is the second person from a minority who has a public position who can realise the strength of their background - the first is fully occupied in breaking down stupid stereotypes).

Now, after the terrible events in New Zealand, the media discussions in New Zealand and Australia include aspects such as the hate and fear that lies at John Howard's door, and acts of compassion and support (solidarity) with our Muslim neighbours have occurred.

To be sure, lobbies have become better at identifying and addressing the real issues underlying such events, and Ms Ardern has taken heed of their lessons, but, in my opinion, the main difference has been the presence of an authentic  leader, a woman who is unafraid to talk publicly of love.

Jacinda Ardern, you are magnificent.

I would also like to offer my congratulations to the people of New Zealand for having the insight to elect such a splendid leader, one who is making not just New Zealand, but the whole world a better place.