I should also add that the Sengoku period benefits from a lot of contemporary outside sources as well, like by European missionaries and Chinese/Korean diplomats, that provides a non-Japanese viewpoint. For the Three Kingdoms period this is not really a thing, since the Korean and Vietnamese records about this period are written much later and are of dubious authenticity.

I will say, however, the historiography for the Three Kingdoms period is much clearer than the Sengoku period precisely because we have so few to work with. Once you dispel of the notion that the stories from the novel Romance of the Three Kingdom are in any shape or form actual history (however culturally relevant they may be), most of the historical records about the Three Kingdoms can be considered reliable. Whereas for the Sengoku period, there is a lot of fiction from the Edo period that got passed down as fact, and only recently (in the West, at least) is there a genuine effort to try to separate the fact (contemporary records) from fiction (later embellishments). It is this difficulty that I suspect is why there is such a lack of information on places like Wikipedia.

This is only true in popular culture, not for academics (at least not those who can read Japanese and Chinese respectively).

We have a lot more on the Sengoku era than we could ever hope to get from the Three Kingdoms. Almost everything for Three Kingdoms have to come out of Sanguozhi, Pei's annotations of it, and Zizhi Tongjian.

For the Sengoku we have tons of surviving documentations, from law codes to prayers to letters. And people in the late Sengoku and Edo were very eager to write down (and often exaggerate or outright make up) the deeds of their ancestors. We have so many chronicles it's a huge effort to read all the relevant ones and compare and contrast what they say with each other (though not something historians shy away from).

While not saying the info contained are correct, take a look at the Chinese wikipedia pages of Xiahou Dun and Xiahou Yuan. We don't even know who their fathers are or how they're actually related to each other. All Chen Shou tells us is they're of the same clan and Yuan is the younger of the two. Compare that with the compare the length of Imagawa Yoshimoto's wikipedia page in Japanese and all the subpages that link from it.

I'm not familiar with the Sengoku era and historiography but I have some knowledge of the three kingdoms era and the SGZ.

In terms of why Three Kingdoms period may have so much translated: Professionally, not that much, the ZZTJ has been, Empress and Consorts by Robert Cutter and Bill Cromwell for the female biographies, Bill Cromwell has done Liu Bei, Liu Yan, Liu Zhang and segment nine of Shu's SGZ Dong He, Liu Ba, Ma Liang, Chen Zhen, Dong Yun, and Lü Yi as part of a project that once intended to translate all the SGZ. There are a fair few works like Michael Farmer's work on Qiao Zhou or Rafe De Crespigny's Generals of the South that provide a lot of information and quotes from the SGZ but aren't direct translations

However, with the game franchise Dynasty Warriors getting people interested in the era, people enjoying the novel then going onto SGZ, sites like Kongming.net had amateur SGZ translations and in recent years Yang Zhengyuan has been translating the SGZ. These are providing a lot of people easy to access English sources. There have also been the odd translations elsewhere of HHS and Jinshu biographies of three kingdoms figures though that is very rare still, the focus is very much on the SGZ.

From what others here have said, there has been more translation work, helped by there being one source vs plenty from Sengoku era.

Something that may perhaps have lead to a misleading sense of the SGZ is the idea of Xiahou Dun and Xiahou Yuan as small generals. Relatives of the Wei dynasty who fought so put in the family section (SGZ 9), both serving Cao Cao for a long time. Xiahou Yuan rose to be Cao Cao's commander in the west with a staff of authority, Xiahou Dun may have lacked that military talent but he was given extraordinary favour by Cao Cao and was Cao Pi's General-in-Chief. Both men were placed in the imperial temple, it is not surprising both men had full biographies from Wei scholars and compiled by Shu-Jin officer Chen Shou. Other figures did not get such treatment.

There are major figures without an SGZ so for whom we only get glimpses. Powerful females like Sun Luban and Luyu or their aunt Lady Sun, as they were not Empresses, get no biography. Or figures left out for political reasons, the Ding family who backed Cao Zhi over eventual Emperor do not get their own SGZ and Wu Chancellor Sun Shao may have held the highest rank in Wu but we know next to nothing about him due to internal Wu dynamics.

Rival warlords to the main three kingdoms do not always get big biographies, Yuan Shu, for example, may have been a southern regional power and a false Emperor but even with annotations it is very bare-bones (also very negative) and none of the rival warlords gets the same amount of detail as the rulers of the three kingdoms themselves. Details about the administration of rival warlords can be lacking, they (like Yuan Shao at Guandu) can be painted very much by their rivals to reflect back on the faction writing about them and senior generals like Liu Biao's southern commander Huang Zu only get mentioned when getting defeated by the Wu forces.

Then there is Shu, one of the three kingdoms, who never put in similar history projects (bar a local one that got abandoned amidst punch ups) as their rivals and so details are sparse. One of the few Shu generals to be given a posthumous title and temple Huang Zhong barely gets five paragraphs (one of which is, he died and what happens to son), Liu Shan's two Empresses combined get barely four paragraphs and so on and so forth.

So the SGZ, though Chen Shou's work is admired and Pei Songzhi's work has done major gap filling, does have large gaps within it and very few sources to provide a bit more detail or a counter. The HHS does sometimes provide such a counter (Sun Ce vs Lu Kang, Cao Cao at Liyang) where the SGZ records may not be correct and modern historians can point to issues within the texts like the way the Cao Shuang regime is portrayed but there isn't an alternative source. The SGZ is a good work of history but that and the ZZTJ (with some from the HHS and Jinshu) are the only sources we have.