Because one can be pinned down to government involvement, while the other can't.

It's the classic CCP defense: "We're not an authoritarian country, you're just racist!".

EDIT: not saying that it was or wasn't a lab leak, but as soon as the government shoots down the possibility with "you're racist", I immediately get suspicious. The same way that I get suspicious when Israel shoots down criticism with "you're just anti-Semitic". It's basically gaslighting with identity politics.