I think that NBN has become less used due to the National Broadband Network (NBN), hence why they only promote their nightly news as NBN.
Allowing NBN co to go under NBN always had the potential for a negative view. This is probably part of the reason for NBN mainly having Nine branding instead of NBN. The other factor is probably cost. I have noticed Prime7 at times just use default 7 branding on the promotions for programs eg Home and Away etc