^^ Conan wasn't properly advertised and even if it was, there were no 'star draws' to it. We may know Momoa from SG:A, but the average movie viewer has no clue who is this skinny guy and why is he playing Conan. Public perception is that Arnold is still Conan even people who never watched that movie! So there.
I wish I was as skinny as Momoa!

But I do agree to a certain extent. However, if you look at another recent fantasy remake, the 2010 Clash of the Titans, there wasn't much for star power in that film either. Sam Worthington was in Avatar, but I doubt most people remember he was. Liam Neeson was the film's only major star, and he had what was essentially a supporting role.