The reason why they Cover blocks any collab between the two ENbranch is simply look at twitter and look at Luxiem's effect on the previous female waves.
Niji can live off of Fujo and Shipper money while cover cannot due to their idol image and either by association of all the "thristing" "Shipbaiting' and "Schumerism" made by Nijis would lead to a influx of unaware gachis and idol purist schizo leaving in fear of nijisanji's sex joke brain rot and menhera.