Why Sea Monsters died out is still a mystery. Some scientists think it is because of climate change. The earth became cooler, and the poles expanded. The sea monsters couldn't adapt to this sudden change of climate, and died out.
Another explanation is because of mass extinctions, such as the one that killed the dinosaurs. Scientists think sea monsters died from things like supervolcanos, asteroids, and other space rocks, or megatsunamis.
A last explanation is that new creatures evolved, and killed most of the ones that were dominant before. An example of this is the megalodon. Some scientists think during the time of megalodon, killer whales evolved, and due to their numbers, and superior brains, killed them. Scientists think this is what happened to other sea monsters.
Scientists also think it was because of migration. Many of the sea monsters prey were migrating such as whales for megalodons, and livyatans, and plesiosaurs for pliosaurs. Food became very scarce, and eventually killed the sea monsters.