While we're on the topic I'm surprised Microsoft hasn't already shoved their entire FPS catalog of heroes into Call of Duty. Master Halo in COD could probably make way more money than an actual Halo game could.
Brian Bloom already does a shitload for COD so getting him to hop on the mic to record a few lines for BJ Blazkowicz would be trivial. Doomguy doesn't even talk.
Maybe they're saving it for BO7, fits better with the near future scifi theme even if BO6's 1990s setting is prime for golden age FPS heroes.