Battleworld would be a hot mess as a permanent setting, requiring massive amounts of exposition about this strange mixed up world in every issue of every series, and would disallow lots of the types of stories writers like to tell in the Marvel Universe. It might be a neat place to visit, but you wouldn't want to base your main comics line on it.
And an MCU line would be problematic for the actual MCU if allowed to be too big, besides excluding at least two major franchises that make great use of Earth as a setting, Spider-Man and the X-Men. Besides, the MCU Earth is already one of the alternate realities getting destroyed in Everything Ends, with a chunk possibly incorporated into Battleworld, so if they publish more comics set there, then they are recreating at least one of their regular universes.