The impact of a sufficiently large asteroid or comet is one of the theories that have been put forward to explain why dinosaurs, as well as many other plant and animal species, became extinct 65 million years ago. why would this extinction-level event have had such a major impact on life on earth?

Because the biodiversity decreases.

Explanation:

An ecosystem is richer when it has a higher biodiversity, remember that each living thing has an specific role in the food web. When in an ecosystem this web is bigger, increases the probability of sustain a living thing. For example, in the sea, if there is a fish, this is going to be more probabilities of survive if this fish can eat different plants or other fish. But if in the ecosystem there are not a variety of them, it probability to survive is less if one of it food desapear.