For the vast majority of movies, that very first trailer is the most important piece of marketing they'll send out into the world.
Those initial glimpses at the film will introduce audiences to the tone and style the director is going for, and if they don't like what they see, then a lot of people might not even bother watching the second trailer, let alone pay to see the film itself.
Important as they are, these early teasers and trailers are also at a slight disadvantage within the grander marketing timeline. Because they come out months - sometimes even a whole year - before the films they're representing, a lot can change in that time, and they can quite often include footage that doesn't make the final cut.
While scenes from the trailer not making it into the film is a common occurrence in this day and age, we very rarely hear the creators chime in and discuss exactly why this happens, but in the following cases, we were offered some solid reasons, directly from the mouths (and keyboards) of the filmmakers themselves.