Why? Because it seems like US institutions and people (right up to Fauci) were involved in this research and may not want the domestic blowback.
Conveniently the CCP don't want a paper trail either.
I'd be pretty sure the various scenarios have already been gamed out in both countries.
Edit: Not sure why this is being downvoted, but just in case it’s a reflex because I mentioned Fauci: yes, he was head of NIAID, and yes, the NIH did fund this type of research at the WIV. The grants are public information.