I can't fathom why the show's fans don't like the movie. Were they expecting more campy comedy? Considering the subject matter, I don't know how the movie could have been done better. It's a perfect prequel.
I think it's mostly because almost none of the characters from the show are in the movie (and a major part was re-cast, and nobody likes when that happens). Yeah the show has a lot of campiness but that's a core part of the charm that people see in the show. The movie is resolutely NOT charming. The show was funny and scary and moving, and even though it was one of the darkest shows on TV for that time, it was bright and cheery compared to the devastating darkness of the film.
The film is also really strange in its structure, which adds to how disturbing it is. The whole first 20-30 minutes feels like this kooky comedy and then there's a radical shift and descent into some of the darkest stuff I've ever seen in a movie.
The TV show was pretty much mainstream and a little left of center. Also another part of the charm of the show was that every week you'd get to see all the characters in this town and you'd get this warm feeling, like you're catching up with your buddies. For those fans, watching Fire Walk With Me was like expecting to see your old friends once again and instead being thrown into a madhouse and told that you'd never see your friends again.
I can see how starting with Fire Walk With Me would be a very different experience, because you'd be going from this really bold piece of cinema to a relatively mainstream show with flashes of brilliance. I could see how that would be disappointing.
It's also definitely fair to call the TV show uneven. I don't think there are a lot of people who would defend every part of the show, especially in the second season. Even Lynch admitted that the second season didn't go the way they had wished. But it was still so fun to watch and I love it for its overall charm and flashes of brilliance. It just has to be said that the movie is a totally different animal.