Because it's nice to see a movie where the heroine looks like she kicks some butt. Granted, I haven't read the books, but Prim seems to be a girl that's very much wanting to stand on her own two feet. This is pretty refreshing when you consider that female characters are usually love interests first and assuming they're allowed to be as such, take-charge females second. What's scary is how often the female lead is made into one of these characters regardless of what her original role was in the books.
Don't believe me? Look at adaptations of well known books. Compare the roles of the book and movie characters. In Jurassic Park the character of Ellie is made into a love interest for the male lead whereas in the book she was just one of Grant's coworkers. While this was done to give her more screen time, I couldn't help but get frustrated that Hollywood was only willing/able to give her screen time if she was hot for the bod of one of the main characters. Even in movies where the female leads are pretty tough, they're almost always played down in favor of the male characters. For example, Alexa Vega in Spy Kids was portrayed as a take charge character, but seemed to play second fiddle to Daryl Sabara's character in most of the movies. (She had a more dominant role in the first one, but Sabara seemed to be the main focus.)
Seriously Hollywood, what gives? Women watch movies. Women love buying products. We don't mind romances, but why is it that every female character in action/suspense/supernatural (any movie that isn't a comedy or drama) movies tend to be love interests or weaker secondary characters? Why can't we have a mainstream movie where the female characters are just as kick butt (if not more so) than the male leads? I'm not asking for you to have every movie to have female characters that are non-romantic interests and are Rambina incarnate, just to have movies where the female characters are tough, aren't played down in favor of male characters, and occasionally can play alongside the men without having to boink/smooch/lust after one of them in order to fit in with the group.
That's why I'm so glad to see this movie being made. I'm aware that Prim has a love interest in the movie and I want to stress that I'm not anti-romance in movies. I'm just glad that the movie seems to have a female character that appears to be the focus of attention and is just as tough as the guys are. It sets a valuable example for young women in general: that we can be just as good as guys are, if not better.