From what I have watched on documentaries,
It is not one thing, as others have said on this thread it is many factors ...
There was a volcano eruption which changed the world climate,which was due to the ash blocking the sun.
This in turn would have led to a HUGE climatic change,no sunlight so probably many plant life would have died which would have killed the herbivores and which in turn would spread diseases due to mass death.
Now the Carnivores will have little to eat and being diseased might have killed the last of them if the cold climates did not.
There are evidences of asteroids falling to earth,but most think that it may not have killed all of the dinosaurs.
I am no expert, it is just a scenario I built around the info I had come across from different sources.