Won't happen because it's one step closer to a monopoly. Comcast bought out NBC Universal a couple of years ago, so you can bet there are going to be eyes on the deal, and the feds will nix it for anti-competitive practices.
The Comcast board has also made a few enemies in Washington (over the NBC deal), and grudges last a lifetime there.
In addition, you can bet the satellite industry will step in to block it, because it won't take a genius to see how this merger could disrupt their business by offering cut-throat prices to lure customers away, collapse the satellite business, then raise rates.
The government should have left AT&T alone.