Its Energy
When calculating the strength of the asteroid when it hit the earth, it was estimated that it had 21 to 921 billion times the amount of energy that the famous atomic bomb that hit Hiroshima in World War II did.
This means that the impact would’ve destroyed everything in a split second. The dinosaurs had no chance, but lucky, it happened so fast that they didn’t feel a thing. They probably only saw a flash of light followed by complete silence.