Take a look at what the Bible - the best source we have for this - says. God did it because wickedness increased throughout the world beyond a certain point.

Science is showing us that there is structure in nature. Science is showing us that nature adapts to certain structural changes. Science also shows us that nature collapses in areas where the structural changes are too great. The science of psychology shows us that people go mad if their psychological structure gets too far out of whack from what it should be naturally. So, why is science so unwilling to look at the spiritual, structures in the universe? It's in spiritual structures that we see hints of God.