People need to realise that trailers are little movies that have to be made, that visual effects have to be finished etc.
It's not just the "bad marketing guys that won't release stuff", and Covenant is probably pretty visual effects heavy and they want to show some of it's scale in the first trailer, so that it will have a greater impact on people.
I'm sure they will show some footage soon, but people were like "they can show a trailer" when they basically just finished filming back in August and that's just ridiculous and not how it works.