Robin Williams is a big star but he wasn't relevant of late so I doubt NBC and CBS will broadcast specials. Why is ABC doing a special and not the others?
1. ABC has enough footage and personalities to do specials like this
2. Robin Williams got his start on ABC through "Mork and Mindy"
3. ABC has no hit shows in the summer like CBS (Under The Dome) and NBC (America's Got Talent) do so they have room to preempt
4. Ben Sherwood controls the network now (or he's about to soon) and I'm sure he's willing to give the news division as many specials and shows in primetime as they want.
Between GMA, World News, World News Now, Nightline, 20/20, 20/20 Weekend, GMA Weekend, World News Weekend, NY Med, WWYD?, and their Special Reports which interrupt programs at times when NBC and CBS don't; ABC probably broadcasts the most news programs out of all the broadcasts networks. (it's arguable whether their news is really hard news or softcore, female-oriented, showbiz news but it's still news)
Not to mention the O&Os have expanded their afternoon newscasts.