Either your going to believe in miracles or your not, I personally do...
But why does not God prevent "all" tradgedies from happening? He does prevent some in some cases and not in others...? Why, you ask?
First off, God has a very different perspective on death than we do, he might allow someone to be killed at an early age, for example, so that they might "return to him" still in a pure, unblemished state, or maybe just because he has a different perspective on death, as he sees it as leaving a sinful, ugly, hurtful and hateful dying world, and just as a "returning to him", which is not that big a deal, from his perspective, he might even see it as putting an end to their suffering and taking them to a peaceful, blissful existence with him...
God might allow a tradgedy in this world to happen for any number of reasons, reasons that I am sure is part of his "larger picture" bigger plan... I know part of that plan is bring humankind down low, to eliminate their pride and arrogance and large, inflated ego's, and get to a place where all of humankind admits, they cannot govern themselves, and need to make all humankind lowly and humble, in order to be prepared like a table for the Heavenly one, for them to allow him to rule, willingly, to "cast their crowns" at his feet as it were, and this may take the allowing by God of many tragic occurrences in this for this to happen...
This period of time in which God is going to allow this to happen is known as the great tribulation, those who die, before this time begins occurring, are getting a "blessing in disguise, as they are only going to God, and will not have to suffer through the great tribulation period, but this point, this period of time (and this is what I know) is meant to bring humankind low, and admit, they cannot govern themselves and are in need of a heavenly king (J.C.) to rule earth...
Those are my thoughts anyways, all tradgedy and suffering that God allows, (for he does prevent some, yet not all) But we human beings either want to say since God does not prevent all tradgedy or suffering, he is bad, rather than mentioning the fact that he does step in and prevent some, and he is therefore good, even if he does not prevent it "all") But anyways, it is my belief that the suffering and tradgedy that he does allow he is working in/out for the part of his "bigger plan"
Anyways, those are my thoughts,
God Bless!