I don't think that is quite accurate. It's top rated by people who vote on Yahoo Movies, but if you head off to websites that report earnings, Serenity is in trouble if the hardcore fans don't go see it an extra 10 times. It's already fallen to the 8th spot in America and has only grossed 17 million. Sure, it may be a better movie than Flight Plan, but executives don't judge movies by their artistic merit. FP has grossed over 53 million in two weeks, a number which may just ensure the survival of the Firefly IP.
But 17 million isn't even half of the 39 million spent on the movie. I kind of wonder what the hell they spent 39 million on... The actors are all cheap TV level players and the 3D effects were ok, costumes were bland... But anyways, they still have a lot more tickets, DVD sales, merchandising to sell to recoup their losses. 'Normal' people just aren't interested in a sci-fi film they've never heard of before. I guess they would rather watch Jodie Foster panic on an airplane.