What the title says really. And the answer definitely isn't "because fanfic/fandom is bad/dumb", let's get that out of the way.
So, my mind was wandering and I got to thinking in particular about hanahaki disease, and how despite all the melodrama and romance possiblities, it's remained thoroughly a fandom trope/plot, and hasn't shown much movement into mainstream like some other tropes (like say omegaverse, that merged in some extend with mainstream things like shifter/werewolf romance ect).
For those who don't know, hanahaki is a very popular fandom plot/tropes, particularly for angst, although occassionally other genres (I've read a few that play it up for comedy or just as part of the adventure, although I can see how that would be difficult), very popular across fandom, where unrequited love leads to flowers growing in your lungs, coughing up petals, and eventual death. There's a lot of variation, as with all tropes, like whether the love must be actually unrequited, if it's enough to simply believe it's unrequited, if it can be cured, if there are way around it (a common one is surgery to remove the flowers, also removing your feelings for that person), if you can get it more than once (for the same person?), if it's actually deadly, if naturally getting over the feelings sees it pass naturally, and if it's actually the love that causes it or if it's the bottling up of your feelings.
Another thing would be soulmate tropes, things like matching soulmate marks, you first words written on you somewhere, seeing through each other's eyes, feeling each other's pain, there's so many variations that can lead to fascinating world building, and are also generally (the ones that art interested in such and not just fluff) fantastic at looking at things like sexuality, platonic soulmates, polyamory, pre-destined love vs choice ect. But, it's another that you rarely see move into real life.
And fromt all that, I started thinking about why certain things like these remain pretty much entirely fanfic/fandom tropes. Because they've got a lot of potential. There's a lot that you could do with these things. So, anyone got any thoughts? Any ideas why? Or any other tropes you think remain fandom tropes that have a lot of interesting potential?