I'm gonna be honest, I've always been kind of surprised how many tragic endings show up in the realm of LBT fanfics. @OwlsCantRead I'm kinda not sure what you mean by "blame the source material," either. The first movie is the only one that gets very dark, and it still has a happy ending. 
I should elaborate, oops. That
is actually why. It is actually the disparity of the
sequels being softer and not as harsh as the original that causes authors to recently start exploring the darker parts of the franchise, by using the lore and world-building of the sequels and combining it with the tone of the first film (and then some).
Now I admit, this was a shift that was perpetuated by Rhombus'
The Seven Hunters, a fic which got many of us into the fandom, or ended up inspiring others who wrote stories which in a cyclical cycle of inspiration, starting the current generation of authors. Now I'm not saying he was responsible, but he was a catalyst to the current tone, direction, and focus of many of us. That's why many of us focus less on slice-of-life and more on character-driven, often somber stories.
Oh, well, in any case, I guess this makes me a rare LBT writer who actually tends to write *gasp* happy endings! Sorry to let you guys down... 
That said... most of us still like and prefer happy (or at least bittersweet) endings, me included. While there are a lot of darker, angsty, and somber stories, there
aren't actually a lot of fics that flat-out end in tragedy.
As much as we write things being depressing, they tend to end on a sweeter note. Those that do (the 'kill the gang' stories, gotta have at least one) tend to stick out as a result, haha.
