Writing poetry is a lot harder than it used to be; which doesn't stop everyone from writing poetry, but it does stop everyone from reading it. The four very sad poems by Christina Rossetti provide a clue as to why. Three of them are on the same subject -- how sad Death is. Or, more precisely, how sad it's going to be for you when I am dead:
WHEN I am dead, my dearest,Yeah, yeah, she's saying don't be sad, but the "thou"s and the "wilt"s and the simple-dying-fall of the last two lines give her away. My dad directed that his funeral service end with Dixieland music, and it got a laugh. That's a valediction forbidding mourning, but this is someone saying "I couldn't have another piece of pie" while holding our their plate for more.
Sing no sad songs for me;
Plant thou no roses at my head,
Nor shady cypress tree:
Be the green grass above me
With showers and dewdrops wet;
And if thou wilt, remember,
And if thou wilt, forget.
This you'll-be-sad-when-I'm-gone is kind of an adolescent emotion. Which is to say that we're stuck with it all our lives. The reason life imitates high school is that we have the same emotional response to everything that we did in high school, we've just learned to manage it better. (Or not.) Plus we're distracted from concentrating on the overwhelming richness of our emotional lives by mortgages and bad knees.
How does this relate to poetry's decline as a cultural force? Because pop music and episodes of hourlong television are better delivery systems for this kind of emotion. Not that poetry is obsolete -- it's still fun to ride horses (I am told), and there are still people here and there who believe in the Bill of Rights. But all three things aren't as ubiquitous as they used to be.
I might add that this poem, about a murder-suicide of a British couple in the face of the 1857 Sepoy Mutiny, shows the romantic side of imperialism in an interesting way.