What if Comcast buys Viacom?

Instead of Comcast buying 21st Century Fox (if Disney now owns Fox), why can't they buy National Amusements (including Viacom and CBS Corporation)?

Media
Comcast's acquisition of National Amusements for a NBCUniversal-Viacom merger, NBCUniversal will own Viacom's various companies and networks.

Film studios that will be owned by NBCUniversal

 * Paramount Pictures
 * Nickelodeon Movies
 * Insurge Pictures
 * Paramount Animation
 * MTV Films
 * United International Pictures (50%)

Television channels and studios that will be owned by NBCUniversal

 * MTV Original Productions
 * Paramount Network
 * Showtime Networks
 * Nickelodeon Productions
 * Nickelodeon Animation Studio