...except the Alien is one of the most recognized creatures in film. Virtually no one who might be remotely interested is going to see Alien and think "oh, just another monster film."
And Xenomorphine is right. While that Prometheus poster may be artistically inspired and/or interesting to look at, it doesn't actually resonate with the film itself or have a thematic connection. Even the font used for the lettering is off (that is a different matter though.)