Whedon didn't need to kill off 2 great charectors and pretty much end the story with Serenity. Why eascapes me.
I found Serenity disappointing too. There was something very much missing. The best I can describe it is like throughout the series they kept investing in the characters and the background etc, so later shows would be even better. And for the movie they just cashed in. That doesn't quite say it, but it's the best I can do.
It's traditional to write characters out of the plot when their real-life situation will keep them from coming back. If those actors had lined up conflicting work which was likely to make them unavailable if something Firefly showed up later, or if they just didn't want to, then better to make it clear what happened to them in case there is another show.