To measure impact, Nielsen and Sony created a score for each event derived by the number of people who viewed the event live, the number who could recall details about where they were during the occurrence and the number who could remember discussing what happened with others. Those parameters may explain why the surreal, slow-speed chase of O.J. Simpson's white Ford Bronco on June 17, 1994, ranked higher on the list than the 2011 earthquake in Japan, the Columbine High School shootings and the 2010 BP oil spill.
I suppose that's why this is so heavily biased towards recent events. Of course people can remember where they were when Whitney Housten died, that was like 4 months ago!
I don't even think of most of these as TV moments because I heard about or even watched them on the internet.