ProRes is very efficient codec. Being able to export videos from Hitfilm in that codec on Windows as well would make life easier for a majority of users.
Agreed, and up voted, however...
ProRes on windows is a licensed codec. Apple must be paid. It's likely that, if added, this would be Pro-Only. For Express it would be a paid add on, and FXhome recently discontinued a paid codec add on in favor of using OS native codecs.
Prores for further editing and HAP for use with media servers.