Because in the tv show & the film it reference's the Unification War two of the main characters fought in, Goverment (Alliance) vs Browncoat's, Browncoat's lived on the other rim planets which were poorer & more free from rules but the Alliance wanted everybody under one government hence the war between them.
The fans are called Browncoat's since Fox screwed the show, showing it out of order & some days it was on late at night others in the morning leading to its quick cancellation but the fans stuck with the show not giving up on it resulting in the 14 episode series being released on DVD. Joss Whedon creator of the show called us the Browncoat's since we shared many similarities to the Browncoat's (us) & the Alliance (Fox). We along with enormous sales of Firefly (currently the 2nd biggest DVD ever on amazon.com) & Joss Whedon's love for Firefly got Universal to make a follow up film called Serenity which continued the story along with the cast. Though the film didn't go great at the Box Office it has had huge consist ant sales on DVD resulting in Universal releasing a Colloctor's Edition last year in a possible attempt to see if people will still buy it, considering a sequel. And Fox surprisingly announced Firefly on Blu-Ray & it was leaked a Special Edition of Firefly is on its way this year too. There has also been an extremely popular comic series of Serenity in 2006 which was the 4th best selling Graphic Novel of that year, & the last comic of the new series is coming out in a few days, & got a sin off comic coming next year too.
There is more about us but I won't bore you that much, already tried to condense it down as little as possible

So fingers crossed that all this stuff might get us a Sequel to Serenity.