Because it's considered "vertical integration" and therefore not subject to anti-trust laws. Comcast merging with Time Warner would be considered horizontal; Comcast merging with NBCU was considered vertical and therefore allowed to go forward in January 2011.

I'm currently reading Susan Crawford's book Captive Audience, which is a detailed history of this exact topic. Recommended. Here's a talk she gave at Harvard summarizing the story and issues at stake: http://www.youtube.com/watch?v=7R4xhwy-1oI