There are many science fiction/fantasy TV series where several/many main characters are killed (A Game of Thrones, Firefly / Serenity, Earth: Final Conflict, Robin Hood, even back in Blake's 7 days). It seems like this is more common than in other genre.
Do more main characters die in science-fiction/fantasy TV shows than in other genre?
I'm afraid I can't offer you a study, but I can offer two plausible explanations for the phenomenon on TV.
Killing off main characters is a risky business in TV drama. Audiences stay faithful because they identify with particular characters, and generally death scenes put them off and may cause them to stop watching. This is actually a shame because (a) new character dynamics extend the life of the show and (b) death makes for great drama.
The way in which killing off a character in Speculative Fiction differs from mainstream fiction is that it's actually not that much of a barrier to their return. In mainstream fiction, dead is dead, and there are no get out clauses. Pretty much the only way you can bring a dead character back is in flashback. With SF, they can be ghosts, resurrected by McGuffins, time travellers, or any of hundreds of other fantastic possibilities. Killing off main characters is therefore lower risk in SF than in other genres, in that, if the ratings take a hit, they can be brought back. Even if this is the writer's intention before their demise, it can be used to create buzz for the show. Examples of this are Buffy in Buffy, and Starbuck in new Battlestar Galactica.
The second reason is that, possibly because of the first reason, it has become more acceptable for this to happen in SF. I mostly lay this at the door of Joss Whedon, who has never been afraid to kill off a character if it was dramatically appropriate (although he may have sometimes been more keen to bring them back than perhaps I'd like). Possibly the best ever use of this technique was (spoilers for Angel)
Doyle, who was established as a Main Character for a full half season before dying in a thoroughly shocking moment of self-sacrifice.
I'd argue that the emotional power of this particular death created an environment in which TV executives understood that fans and viewers could appreciate the higher stakes drama of losing a main character. It's not that this wouldn't work just as well in mainstream drama, merely that there hasn't been a high profile enough show that's taken a similar risk in such a massively successful way. Studio executives are a pretty conservative bunch, and the power of having an example to point to and say "look how much this increased ratings" is not to be underestimated.
Scifi/fantasy shows have always been slightly different from other shows. They have a specific audience, more of a cult, watching them. The shows themselves then push the limits by taking the necessary risks of presenting cliffhanger with character deaths to get more audience, and to keep old fans watching. In essence, they can afford to kill off characters without paying for it, because the fans stay true to the show no matter what happens.
Unlike scifi/fantasy shows, other show fans stay true to a specific set of characters that sometimes last until the end of the series. These shows also have simpler plots, so they are easier to follow.
Scifi/fantasy shows, more often than not, get canceled because of plot or character death/changes. We are used to seeing characters die only to be brought back to life again, sometimes during the same episode.
There are cases in drama shows, like the medical drama show Grey's Anatomy, where during the same episode, one main character dies while another main character is about to die, but heals and leaves the show.
So in other genres like drama, it's also common to kill characters and replace them with new ones.
In Sitcoms, main characters rarely die (because the show is about them), but it happens. For example, Charlie Harper on Two and a Half Men(whatever the reason). There are many other cases like 8 Simple Rules, etc.
Characters of scifi/fantasy shows also die more often because many of them are based on a written story (a book or a play).
For example, A Game of Thrones follows a series of books called The Song Of Ice And Fire in which character die regularly being replaced by new ones.
The shows that don't follow a book, have more complicated plots and are based on hostile environments, leading in the death of their characters. It also gives the show an edge which attracts new audiences.
Probably, one of the main reasons that scifi/fantasy shows can afford to kill their characters, is because they can bring them back to life by some means or power, unlike other shows where a character dies and cannot be brought to life.
During the scifi show Battlestar Galactica, each Cylon character dies multiple times in each season. How many times does one character die?
(please spare me the comments about Cylon death explanation)
