>>5357859Well, first it's important to note that not all of the dinosaurs died. In fact, one type of theropod has become very successful in the modern era: Birds.
But if you mean why did all of the other large dinosaurs die? Well, the prevailing theory is that most of the large dinosaurs died out as a result of something called the K-T Extinction Event around 66 million years ago. A massive asteroid (6-9 miles wide) impacted on the Earth in what is now the Gulf of Mexico. This devastated the global environment by throwing so much material into the atmosphere that it created a global winter which interrupted the photosynthetic cycles of many plants and planktons that formed the basis of the foodchain. Without these plants, the large herbivorous dinosaurs died. And without large herbivorous dinosaurs, large carnivorous dinosaurs could not find enough prey to satisfy their dietary needs and died out as well. Only the smallest land animals survived this monumental event.
In the years that followed, mammals would grow to fill the niches once filled by large dinosaurs, and dinosaurs would continue to evolve and refine themselves into the modern birds we see today (with some aberrations, like the South American Terrorbird).
Hope that was helpful.
