Or why didn't He stop the carnage in the First World War, or that wreaked by Genghis Khan, or by the Spanish Conquistadors, or by the Catholic Inquisition?
Every time something dreadful like Sri Lanka occurs, people come out with this question. History would give you the answer. Go and study human history and our constant carnage, in every generation, in every society and waken up to the fact that evil is in all of us - some of us vent it with carnage like that. Others vent it with murder. Yet others vent it with sexually abusing other people and children - that, too, is a form of carnage on individuals. Why should we only pay attention when there are hundreds (or thousands, or millions) of people destroyed by human carnage? We are all creating carnage on this beautiful world by our consumerism and greed and disregard for other living creatures.
God gave us all we could need or want then handed over management of animals, the planet and our families to us. He does not take that back. He does not start to micro-manage the planet He gave to us. We mess up. We suffer the consequences. Innocent people suffer the consequences. All due to our sinful, rebellion against doing things God's way. Don't forget that if God starts to stop people doing some things, He will stop you from doing some bad things too. Oh, but it's only ever "other people" that we protest about. We never think of what God would stop us from doing, yet evil lurks in us too. We lie. We steal. We are immoral. We are selfish. We are greedy. We try to control others, in our own ways, to get what we want. That's what those evil-doers did in Sri Lanka, just on a vast, noticeable scale. Go to the links if you really want to understand.