DirecTV and Turner Broadcasting don't compete with each other. Making them sell one or the other makes no sense from a monopoly busting/prevention standpoint. If it was a choice between two content creation groups it would make sense.
AT&T has already invested heavily in integrating the DirecTV, DirecTV Now and U-verse TV services. AT&T has made clear that these services are key strategic assets going forward. The DoJ knows this, so it is reasonable to conclude that their goal is to force the sale of Turner Broadcasting. The question becomes why would they want to force this sale. Given Trump's statements it is more than reasonable to conclude that it is to change the ownership of CNN.