Why? So that you can downvote it based not on the content of the trailer, but the assumed leaks, which may not actually give you the full picture aka you are doing downvoting for the sake of it.
Pathetic indeed, just like the so fucking obvious clickbait videos on Youtube which draw in people who then behave like a real hivemind.
Protection of intelectual property, in short. Sure, it sometimes is dumb, but there is a basis for it.