When the UESP switched to the wiki format in 2005, it brought with it a decade's worth of peer-reviewed TES guidance, as well as a well-rounded group of knowledgeable and loyal contributors. When the Elder Scrolls Wikia started circa 2006(?), it had next to nothing, as far as I know. Because the Wikia was desperate for content, many questionable additions went overlooked for years, even into the present day. TES fans typically went to a wikia page, fled in disgust, and found a relatively informative UESP page instead.
Interwoven with this is that TESWiki gets a preferred position in most search engine results (I'm not sure exactly when this started; I assume since around the time the TESWiki started). This search engine manipulation is ultimately a great benefit, as it greatly inflates the TESWiki's traffic. But if you wave a turd in someone's face, they're going to have remarks to make. And for many players looking for TES info, the Wikia was a big fecal log being waved right under their noses. Thus, the site quickly developed a reputation, leading to caustic sentiments like http://elderscrolls.wikia.com/wiki/The_Elder_Scrolls_Wiki?diff=3013&oldid=2835 (clearly he's a female hygiene product, but he got his point across ... ). This general aversion to the TESWiki meant the UESP continued to attract more and better contributors, on average, for much of the last few years.
However, many newcomers familiar with formatting on other Wikias were naturally inclined to contribute to TES Wiki rather than adapt to UESP formatting and slightly more complicated (but superior!) namespace arrangement. There were also just your typical "brain drain" events: a would-be contributor suffers some perceived slight on the UESP, abandons the site, and ultimately goes on to contribute positively somewhere else. With the aforementioned search engine manipulations and network of affiliates, the TES Wiki was the natural place for these alienated people to go.
These factors, combined with the gradual introduction of cosmetically-altered (or outright plagiarized) UESP content, kept TES Wiki afloat.
Really not trying to offend here, just stating a fact. If you think I'm off-base, try comparing revision histories of corresponding pages across wikis. A pattern will emerge, I assure you.
I think the schism over OOG has likely also caused some brain drain from the UESP. Some old debates on the UESP over the matter can be found http://www.uesp.net/wiki/UESPWiki_talk:Lore/Sources_for_Lore_Articles. Contributing to a wiki long-term can become incredibly aggravating for a wide variety of reasons, and sometimes good-natured people Rage Quit in a spectacular fashion. This seems to be what happened circa 2008 in the debate over sources. A lot of UESP contributors wanted all of Kirkbride's work, along with Hogithum Hall, hosted on the wiki and disseminated on the wiki pages. And they wanted all or nothing, unwilling to compromise for the sake of people who disagreed. So they stopped contributing (and probably went to contribute elsewhere).
A side note I find fascinating but which you would probably wish to skip: the ongoing differences in TES coverage by the two wikis are mainly the result of editors preferences,
not policy. On paper, the two wikis approach OOG content in an almost identical fashion (until recently, at least). But UESP editors are just naturally more conservative because the more "liberal" contributors Rage Quit in the days of Oblivion. Most of the major contributors the UESP retained held the same apprehensions over OOG, and they attracted more people who were similarly inclined. It's just cultural; newcomers to both wikis just follow the tone set by others, for the most part.
As mentioned, TESWiki's approach to OOG seems to have changed recently, in that I noticed they've started hosting OOG content directly. Apparently they want to replace not only the UESP, but TIL, too!
Implicit in the UESP's policies on OOG, I think, is that TIL was out there. If TIL had not existed (but the unofficial stuff like Nu-Mantia was floating out there), I believe the UESP's whole approach to unofficial developer content would have been different, perhaps drastically different. I see their roles now as largely complementary, even symbiotic in some respects, rather than in contention with each other. But I don't know if TESWiki's scope is quite as accomodating. It seems to me like the aim at this point is to do everything the other two sites do, just not quite as good as how they do it.
But, hey, at least TIL gets an acknowledgement when TESWiki relies on its content...
Really? I've never seen that ... although God knows that I
should have more than once! Edit -That's the irony of it: as I understand the copyright issues at play, TESWiki could essentially copy-paste entire UESP articles, so long as they give credit. That's all that is required, a cite. I can't imagine how many cumulative years have been spent rewriting UESP articles just to avoid giving this small accomodation.
No offense intended to any individuals with the above. I've met many TESWiki contributors who do not svck.
Ironically, that's actually the only reason any of this irks me at all:
some ...
ALOT of MOST TESWiki editors are really great! I don't mean to denigrate all the fine work which has been done there. It's a testament to how much we all love this series. But I come from this premise: the TES community would be better off as a whole if all the TES wiki-gnomes out there were coordinated on one project, instead of wasting our limited time creating and maintaining duplicative coverage across two sites. Better coverage, a better-informed community, better discussions, and ultimately, hopefully, better games. But that's just me. Peace.