What if Comcast buys Viacom?

Instead of Comcast buying 21st Century Fox (if Disney now owns Fox), why can't they buy Viacom?

Media
Comcast will own Viacom's various companies and networks.

Film studios that will be owned by Comcast

 * Paramount Pictures
 * Nickelodeon Movies
 * Insurge Pictures
 * Paramount Animation
 * MTV Films

Television channels and studios that will be owned by Comcast

 * MTV Original Productions
 * Paramount Network
 * Paramount Kids
 * Paramount Television Animation
 * Nickelodeon Productions
 * Nickelodeon Animation Studio