Apparently they're historically the most successful driver of game pre-orders as far as marketing assets go.
I'd need to see that research. It could just be that games release a CG trailer during the period of marketing that has the largest amount of pre-orders (like now, a game reveal).
Either way, I agree with those harping on the trailer. With the capability for graphics like those in Infamous: Second Son it's about time to use in-game, in-engine assets for game trailers.