MasterMace :
Regulators will block it because it violates Antitrust laws.
They can spin out the rural and other steady-but-lower growth systems to the like of CenturyLink, and others. That's the usual justification, along with claims of all the 'synchronicities' they'll bring the general public (mbleh. . .).
Antitrust laws currently exist to provide a pretext for doing what you want, not for acting on behalf of the claimed reason. The principals of both companies have historically been big net Democratic backers, not to mention of the same ethnicity. They're insiders (and in a radically different strategic position from a Dish Network, for instance). Plus, Comcast's NBC News provides leverage because of its policy promotion and CYA silence for this administration. That's the value in top-shelf news, despite its terrible nominal financials. See Bezos & WaPo.
The fun part will be to see if their fellow cronies, eg. AT&T and Verizon, push back. That's where the battle lies on this deal, and the outcome is not certain.