Highly doubt the WWE wants their own audience in a situation where they have to choose between cheering for Cody and cheering for LA Knight. They'd likely pick Knight, and Cody, so far, has perfectly walked the thin line of white meat babyface to prevent being booed. (Even if you don't like Cody, you can't deny it's amazing that he manages to keep the audience behind him considering he's relatively bland and force fed to the audience)
The last thing they want to do is give him a good shove off that thin line, which is what they'd be doing by making him an obstacle for LA Knight.
Also, Naomi and Nia aren't really the same situation. Naomi's work is passable and she doesn't have a lengthy history of hurting people. If Nia could say the same, most people probably wouldn't care that she was brought back.