One major problem that TV Tropes had in earlier times was the very negative tone some tropers would take toward shows that they did not like. This being the Internet, such conflicts were inevitable. The fact of the matter is that people like complaining, and that however many predefined messages are put on the main page, by and large people think tropes are bad things. If a work uses a trope, then it is bad because it is being uncreative. And so on, and so forth.
Initially these negative comments were simply moderated by concerned tropers who deleted this material where they saw it, chief among these being the admins. The honor system, as it were, was much more effective when the site was smaller, but as this kind of negative discussion became more popular it became clear that more elaborate fixes were in order. This was how Darth Wiki came about.
I do not recall the discussion that led to the creation of Darth Wiki- it's entirely possible that there wasn't one, really. It's existed for long enough that it may predate the concept of broad wiki discussion before decisive action. What I do know is that Darth Wiki provided an outlet for people who loved to complain. It must be understood that when people complain on the wiki, they do not do so from a shameful position. They genuinely believe that the show in question was completely terrible and it is their duty to warn others about it.
Darth Wiki was the perfect source through which this frustration could be vented. There were no courtesy regulations or expectations that non-harsh language be used. The dark background made it abundantly clear to people what they were supposed to expect when they entered it. Even the name, "Darth", conferred a sense of darkness that many reveled in. It's no surprise that Darth Wiki became quite popular with those who were familiar with it.
..This, of course, is the relevant caveat that still plagues wiki moderation efforts to this day. Darth Wiki may be popular, but not that many people are aware of it. Even when the icons were added to the tops of pages, its very existence continues to surprise people.
The limited success of Darth Wiki can be explained by a simple credo- people see what they want to see. For people who genuinely wanted and wished for a version of TV Tropes that wasn't so academic in mission statement, who wanted it to just be for nebulously defined "fun" stuff, it was a dream come true. For others, it was an innovation they hadn't even thought to look for. The divide between "fact" and "opinion" on TV Tropes is a very subjective property. A lot of people don't complain as a force of habit- some days they're just in that mood. They may not even notice it- and for them, Darth Wiki is not a place they would think to look for. There's no reason to think that today's "fact" is any more subjective than whatever uncontroversial information was posted yesterday.
A somewhat common response to complaining edits back then was to "take it to Darth Wiki", but this could only go so far once the fact had to be accepted that a lot of people simply didn't want to. Today, even though Darth Wiki is still popular, the phrase isn't used so much as there are other more objective venues that complaints can be generally classified as. Still, success is success, and while Darth Wiki didn't solve all the problems related to negativity, it was an effective base that gave precedence to using namespaces as a partial solution to other similar problems across the wiki. Ultimately it demonstrated a lesson that's true to this day- in dealing with wiki problems any strong solution is helpful, even if it doesn't work perfectly.