There's a lot of reasons, I think.
1) Having seen other shows or books get popular BECAUSE of their grittiness, writers have more courage now to do what they've always wanted to do. Make dark fantasy.
2) I think many people feel fantasy is childish unless it's gritty, because for some weird reason, people equate "dark and gritty" to "mature and real", which, personally, I find that silly.
3) Many writers write for what's "in". If writing is your career, that's the smart thing to do. So, for example, vampires were a huge thing not long ago. If you were able to churn out a vampire book (especially, a vampire SERIES) before the fad was up, then you were likely to make money off of it, or at least gain a following. Now, "fae/fairies" are getting their chance in the spotlight when it comes to fantasy romance. "Fae" are everywhere now.
4) Many writers also take "Anything that does not further the plot should be removed" to heart. Sure, there may be some funny banter...but it's not moving the plot forward.
5) Writers are uncertain if the funny bits they mean to insert would actually be funny, or if it would even fit in the moment now that everything is so dark. Will the reader criticize you for your humor, that just happens not to be to their tastes? Will they say the humor felt awkward considering the circumstances? Then where should the humor be? The balance is difficult to find.
I understand what you mean with needing a breather. I'm the same way. Actually, I cannot, and will not, devote myself to anything that is consistently dark. It simply isn't fun for me. I don't want to see someone under constant duress, nor do I want to see any characters I may come to love constantly being tortured. I don't want to be worried about their deaths all of the sudden, because I'm going to stop reading if there are no characters left to make staying worth it. I don't understand why it's so popular to criticize writers now for not making it a Free For All on characters. Holding death over the characters I love is not exciting me. It's infuriating and I will stop reading/watching, no matter how intriguing the story and world is. Dangling hope in front of me won't work if it's alongside constant death. Boring parts, funny moments, are a must. I completely welcome derailing from the plot here and there to let your characters talk and joke around.
Ultimately, I see no reason to watch a bunch of people being constant victims on screen or in books, and never gain true strength to overcome everything that's thrown at them (because they fall down at the next thing, which is seen as 'necessary' for drama), when there are enough victims in the real world who could use that time being spent on them.
This is coming from someone who's been unknowingly writing dark fantasy since they were 10. And I mean gritty, nasty, violent, horrific dark fantasy. There needs to be some joy.