Patrick Stewart is the star of the series and takes home the biggest paycheck. The audience goes to see him, not some twenty year old marine no one has ever heard of before. This is just the simple fact where a movie isn't going to match reality in this type of situation.
I think that's vastly overrated. People go when the trailer and promotional material interests them, and/or if the word of mouth is good. That that is ENTIRELY independent of the actor. Sure, you can draw a lot people in with big names. But if the film is good as a whole, people will watch it, regardless of how big the names are. Yeah, there are some who go "What? No Picard? I'm out!" but most people say "Woah, holy shit, what a great trailer! I have to see this!"