Noah carried some baby dinosaurs on the ark along with all the other animals. After the flood, the climate changed and the large ones, which depended on lush, swamp-like environment, could not survive.
Smaller ones became more scarce as man populated the earth and only survived in the ocean and large, deep lakes such as Lock Ness or remote swamps such as the huge Likouala in equatorial Africa. Today, they are like the grizzly bear, only a few are alive in remote wildernesses far from civilization.