Since he is the main antagonist of the film he should be in the poster. The only antagonist in the poster is Jango Fett, the secondary antagonist of the film.
He's definitely the bad guy — but even if it's a great character (played by a great actor), don't forget that:
- He doesn't get too much screen time.
- There's an important plot twist revealing his "dark" nature.
So, basically, they didn't want to spoil the movie ;-)
Fett, on the contrary, is far more known and recognizable.