The same reason they all die in real life. The real question is why Hollywood is so terrified of believable consequences in fiction.
Unlike Elf Witch, I watch tv primarily to be engrossed by a plot/story/ collection of characters. If I have fun while doing so thats good to, but I need the dialogue to be on point and for characters not to deviate (the bane of multi writers on a tv show)
Picking a show like Dr Who, I firmly believe that if the concept ever arose in America it probably wouldnt have made it. Dr Who to me has always been about weaving the characters around a scifi story and watching them deal with what the story has to offer.
I do totally agree with previous posters, American tv esp on the Commercial networks seems loathe to kill off characters wheras i find the Cable shows there dont mind doing it. If the plot/story dictates a character should die, then thats what should happen
I watched the UK version of Being Human for the characters now that they are all gone but one I don't feel any desire to keep watching.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.