Why do so many extremely large animals often become extinct?

I've been researching large marine animals. Some of the coolest, most interesting creatures such as the kronosaurus, elasmosaurus, liopleurodon, and possibly even the great megalodon, are extinct. I haven't really done much research on dinosaurs, but I don't understand how ONE comet killed all the dinosaurs on the planet. Although, human civilization would be nothing compared to what it is today if dinosaurs still existed. In fact, humans might not even have survived with dinosaurs. Can we perhaps clone some of these incredible giants to revive them as we are currently working on with the woolly mammoth. Also, I personally think that we should grow a pair and venture down to the deepest, darkest parts of the ocean that have not yet been explored to see if the megalodon still roams. Unfortunately, I cannot provide links to pictures of the creatures I mentioned because the web addresses are extremely long (like the creatures themselves) and they stretch out of the box, causing the text to be cut off.