One of the most interesting phenomena in fanfiction is the great quantity of Darker and Edgier (henceforth DE) works that I saw: I mean, seeing Ed, Edd, and Eddy dropping f-bombs, Pokémon killing themselves in an over-the-top fashion that the only reason why you keep reading it's because it's so over-the-top that you just keep reading because of curiosity rather than actually liking the stuff, Total Drama fanfics dealing with topics like racism and people actively trying to murder other people. In fact, Rule 43 of the internet says that "The more beautiful and pure a thing is, the more satisfying it is to corrupt it". Of course, this is not something seen only in fanfiction; in fact, it's much more prevalent in original works, I wasn't unaware of it. However, fanfiction called my attention for this because the contrast is much more evident, as it's also an experiment that allows comparing the same characters in different scenarios (at least in theory). In fact, there is a reason why tropes like True Art Is Angsty belong to the DE family, so I decided to investigate the raison d'être of this.
(Keep in mind that this is my opinion, which I built based on objective (things I learn in college and will always haunt my life, especially the ones involving methodology) and subjective (opinion and how life experiences built it) factors. And remember to tattoo in your brain the law "CDNIC — CORRELATION DOES NOT IMPLY CAUSALITY!")
And, considering everything I came to the conclusion is that writers will tend to write DE works because it increases the probability of increasing the successfulness of a work. And the successfulness of an work is determined by how popular it becomes and it does seem to have a correlation between successfulness and number of tropes used in a work (for an example, see the page Trope Overdosed, it's virtually impossible to not find a work you love in that list).
So, another thing to have in mind: popularity does not equal quality. Just because a work is popular does not mean that has high quality (this seems to be the usual opinion on Twilight and other snark baits). However, high quality can be a factor in popularity — people who like to read a book/watch a movie for its quality will tell the others about it.
What makes a fiction story good? Due to the fact that everyone is everyone, there might be no objective answer, but still the answer to the question 'what makes a fiction story popular?' might be relatively easier to find an answer, because there seems to be factors that every popular work of fiction shares, it can be said is how the writer builds the universe, its setting, its characters in a way that attracts the attention of the spectator (and making something 'popular' earns much more than making something 'good'). And the tropes play an important role because they are what build the story.
DE enters here because the main feature of DE is that it allows using other tropes, to explore more things like drama, violence, cynicism vs. idealism, issues that are more prevalent in real word, its consequences and outcomes; if is a series, allowing the themes get more mature as their fanbase also matures. DE is a tool, a superior one, in the sense of having lots of subtropes, and the problem of using it is that the wielder has to have some ability to use it, or else it goes wrong, "writers often are too lazy to make use of what most of those words mean, and ending up randomly 'spicing up' a work with gratuitous gore, cursing, and sex." And having an inexperienced worker using a delicate tool tends to produce poor results and guess what fanfiction has no shortage of it? Well, that's one of the points of fanfiction, to be the gateway for inexperienced writers, but I suspect that's one of the reason why fanfiction is so vilified outside, because it makes DE stories just looking like bad taste when the most likely reason is that the writers don't have idea what they're doing, and get too carried up when going into FULL-GORE mode. It needs maturity to deal with mature and delicate things, like putting rape in a story should not be something to be glorified, but it must be a very dark moment for the story (I recomend this essay, and I agree with the first part of his argument). For this kind of situation, I like this quote from C. S. Lewis:
And this quote from Rated M for Money page, "There's something that's called 'adult animation' and it usually means it appeals to adolescent, teenage boys and that's not really adult in my view." Which basically says almost the same thing.
In summary, it must be remembered that DE is a tool, which enable you to use a lot of other tools that may help to increase the popularity and the quality of a work. The trope True Art Is Angsty exists because great writers tend to use the DE tools to explore more dilemmas of the human existence and because the really bad situations are the ones that test the character of people, be it fictional or not, and Sturgeon's Law says that people will miss the point and think that DE automatically makes a work better. It doesn't. There is no magic formula to produce a Nobel-worthy book, Oscar-winning film, you just have to do what they do: put all your effort on it and beyond, MUCH beyond (or just have the right friends in the right places, but even so you'll have a lot of difficult).
Last comment: tropes are also cyclical, if too much works gets DE, DE ends turning into a cliché, opening the way for non-DE works and so goes on.