Sirius/XM is much closer to the same situation than some would care to admit. Competion from Fiber and cable now cover a great deal of the markets, along with Internet delivery. From a regulatory standpoint, it would be hard to approve Sirius/XM without approving DirecTV/Dish.
I think the bigger issue is what benefit it would be to DirecTV. The selling point before was the ability to avoid the costly rollout of locals in all of these markets. They already spent that money, for the most part. The big value proposition last time was buying it quicker/cheaper than you could build it.
I am sure there are still reasons for a merger, just fewer of them.