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)