Dinosaurs have become extinct probably due to following reasons
1- Large meteorite impact that was the size of nearly one forth the moon.
2- Catastrophes like large scale volcanism and earthquake, followed by release of huge amount of co2 and other gases in the atmosphere which led to blocking of the sunlight.
As a result the global temperature dropped and certainly the reptiles(dinosaur) could not efficiently adjust to it due to lack of food, proper oxygen and water.
Hence the planet witnessed a major paleoclimatic change.