I'm going to jump on the sentiment that it's better to ask why American TV and film is so reluctant to kill main characters, have unhappy endings, or carry negative or dark messages. It's art; it should reflect the totality of the human condition.
And, frankly, I hate when I feel like my material has been dumbed down or I'm being pandered to.
OTOH, there are some American shows with a high death rate. The Wire killed many of its more popular characters, and pretty much anything by Joss Whedon is going to have a high body count. Funny how the shows that kill characters tend to be the best ones...
That is very subjective there are a lot of excellent TV shows that didn't kill off characters or did not kill of whole swatches of them.
I don't get how grim and gritty equates to better television or by not killing of characters equates to pandering and dumbed down.
Sometimes what makes a show fun and well written is the by play and chemistry between the major characters. Killing off one of those main characters can mess with the whole chemistry of the show.
A good example of this is Beauty and the Beast, people were invested in the relationship between Vincent and Catherine. When Linda Hamilton left the show the producers killed the character off thinking that Vincent was enough to keep people watching. The ratings sank like a stone.
People bring up Joss Whedon look at Buffy the main characters were always Buffy, Willow, Xander and Giles. Other characters came and went but those four were the heart of the show. He wisely did not kill them off.
Sometimes killing a character can work I personally thought killing Ashley on Sanctuary opened up better story lines. Of course it did cause a lot of Ashley fans to stop watching the show.