We have already seen plenty of other aesthetic changes, such as the shift that Steve Rogers has made to a Nomad-esque character.
If Hawkeye is due for a new costume, and possibly a new role as Ronin (something that many fans have theorized in recent months), then it could just mean that Clint is one of the movie’s critical secrets that need to remain intact for the foreseeable future.
With that in mind, one explanation for his absence from this post and the trailers is that Hawkeye receives visual and personality overhaul in Avengers: Infinity War. If that’s true, then the folks behind the movie want to keep it a secret for now.