The closest explanation comes from their Facebook.
After 13 years, GameTrailers is closing down today.
When GameTrailers was founded in 2002, internet video was a revolutionary concept. YouTube wouldn’t be founded for another three years; consumer live streaming was nine years away. Back then, online game journalism was still dominated by text articles and static imagery. The most prestigious coverage still happened in print. If you wanted to watch a show about video games, you had to watch it on a special TV station that had 10 shows.
GameTrailers helped change all that. Today, game journalism is dominated by video: trailers, live gameplay, Let’s Plays, and news shows. Now there are millions of hours of content available at any time on your PC, your game console, your TV, and your phone.