Answer
Because to make a trailer you have
to finish the film then take bits from it that would interest people enough to watch it. even though it is normally seen first it is the last thing made.
quote... " the term "trailer" comes from their having originally been shown at the end of a film programme. although that practice did not last long, due to patrons tending to leave the theater after the films proper were finished, the name has stuck. trailers have since been shown before the film (or the a-film in a double-bill programme) begins."
check the link below for more info..
graham
