Why would he be on the poster? It’s not like he was in the movie or had anything to do with it OH WAIT HE WAS AND HE DID.
I have this poster on my wall, and it’s one of my favorites. But you know who’s conspicuously absent? Yoda. The makers of this film knew even then that he was one of the best things about this movie, so they wanted to preserve his reveal as much as possible, to the extent of holding back on putting his image on promotional materials. Granted, they later put his face on ROTJ’s VHS cover, but this was years after the sequel came out. My point is that much of the interest for this movie is the question of what Luke looks like after all these years, and so to see him on the poster lessens the impact of seeing him at the end of the film.