Because it's just a basic tactic that belongs in Politics 101: If you can't answer a question and immediately want to disarm someone: Laugh at the question. It'll immediately shame the person who asked into feeling as though they presented a question worthy of being outright mocked, and will likely persuade a good chunk of the audience into believing the same.
It's an incredibly basic tool in the "being an asshole" toolkit, and if you pay even a rudimentary amount of attention to any politician that you want, you'll start to notice that almost every single one of them does it instinctively, and especially during any debates, where you can't go so much as a single question without either politician barking out a laugh before bothering to dodge around the question.