I think that a lot, not all, the people who prefer the series over the movie is that they actually were invested in the characters before the movie came out. I saw the movie with no more knowledge of "Firefly" than that it was a short lived series that Joss created. I wasn't all that upset when Book or Wash died, because I didn't know their history. I knew a little more about River, because her origin was part of the beginning of the movie. I didn't know these people, so, to me, it was just a stand alone production that I saw as something like Roddenberry's original concept of "Star Trek" was supposed to be ("Wagon Train To The Stars," as opposed to "Firefly's " "Space Cowboy") and, as such, I liked it a lot. I like it more now that I have seen the series, which I ordered from Amazon as soon as I came home from watching the movie, but now I have a much better understanding of what was actually going on.
I like the series more, because now I know just who all of the characters are, how they came to be together, what they have gone through as a group, and what the significance of the ship's name and the term "Browncoats" means. As a Southerner, I grew up in a culture that still recalls our own history of a lost rebellion. "Browncoats" is synonymous with "Johnny Rebs." (Please, let's not get into the slavery issue, because that's not my point)
That was my long winded explanation as to why I like the movie because I love the show