>>10109738OP here also, Here's my idea of why they can't truly unify the WWE and Universal Championships yet. For both championships to be truly unified, one must be deactivated much like what happened with US/IC titles and many other titles before (the NXT UK's series of championships are recent examples).
Deactivate the WWE Championship and there goes the years and years of history and Vince, still an executive of WWE, will not like that. Deactivate the Universal Championship however, and there goes Roman Reigns' 1000 days holding it, which is also promoted going into the upcoming Night of Champions.
Basically WWE is stuck in a dilemma about the world title situation. Maybe in NOC or beyond it, they will truly unify and retire the Universal Championship.