Nobody truly knows why or how the dinosaurs really died. This happened 65 million years ago. That is a very long time. There is some evidence in rocks that shows a layer of a certain kind of rock or dust around the world from around that time and this is partly why we think a catastrophic comet or asteroid hit to the Earth might have been the cause. A website at the Smithsonian Institution describes this evidence but the answer is: nobody knows how the dinosaurs died. People have imaginary ideas based on evidence such as that described at the Smithsonian and many other places, but to be perfectly honest we don't really know.