The usual politics and indifference: SNCF is unable to create a ticketing system+frontend clean enough that they can fix and integrate easily to sell cross-country tickets (“Une erreur c’est produite” has been the motto of their user frontend for 20 years already), other technical details are even more preposterous:
ÖBB uses a leased Siemens Vectron locomotive for the Nightjet to Brussels, but that option is not open here – France is pretty much the only standard gauge European country where Vectron is not approved to run.