The reason is that people like it that way. Studies show that roughly 75 per cent of people make their decisions about what movie to go see based solely on the previews, which is why sometimes you get not only two or three teaser trailers to let you know the movie is coming, but the theatrical trailer to let you know the movie is almost here. When the trailer for Arlington Road revealed the film's ending, the celluoid straw in the proverbial camel's back broke. George Lucas even snuck into the theatre when they unveiled the trailer for The Phantom Menace . Don't kid yourself.
Trailers are a serious business.
Listen to Collin Friesen's entire report, which includes the universally-recognizable voice of Don LaFontaine. Don LaFontaine - from the Internet Movie Database. Arlington Road - from the Internet Movie Database.