because it wasn't a vassal state, especially in 40 they still were considered by everyone to be the true france, barely anyone knew about the FFF

you have to understand that vichy was run by nationalists and nazis, the nationalists controlled pretty much everything at the beginning and started gradually loosing power to the nazis who were more than happy to obey germany's orders but the nationalists fully intented to stab germany in the back when their army would have been destroyed in russia

the repeated allied attacks on french assets, landings in french africa and hitler's personnal hate for france meant they all switched to the allies or started their own resistance groups, leaving the nazis fully in power