Because enough people will believe it when they see an asteroid looming towards them on CNN, Fox, whatever. There are plenty of people that deny climate change because it's convenient, either politically or economically. There are considerably fewer people that think the moon landing was faked, or the earth is flat, or pluto and mars and jupiter are actually jewels hanging from threads in a crystal sphere.

An asteroid is a sufficiently "concrete" science that the average person would believe if they see it. You can't "see" climate change, which is part of the issue.