While you wait for an answer, you might be insterested in [this] (https://www.reddit.com/r/AskHistorians/comments/4t21kq/romance_of_the_three_kingdoms_was_historicalshu/) answer by u/_dk, especially this part:

About Xi Zuochi though, he was actually from the Jin dynasty, which is close enough to the end of the Three Kingdoms for him to be able to write a reasonably accurate history of the Three Kingdoms. It was said that he wrote the pro-Shu Hanjin Chunqiu not to please his boss, but instead to remonstrate against him since his boss Huan Wen was about to pull a Cao Cao and take the throne from the Jin dynasty. So Xi Zuochi went to demonstrate, using Wei as a negative example, that even if you underwent the ritual to receive the throne, that didn't make you legitimate. Since that was his goal, his writing immediately becomes suspect as a historical work, and later commentators and compilers were quick to point out that the Hanjin Chunqiu carries events that other sources didn't, or was portrayed differently. Xi could have made those stories up, but also consider this: Chen Shou could not be too anti-Wei in his Records of the Three Kingdoms because the Jin succeeded the Wei - attacking Wei's legitimacy would indirectly attack Jin as well. Also, Chen Shou may have chosen not to write of the things that the Sima family had done against the Cao family since he could not make the Sima family look like usurpers. Xi Zuochi, by denouncing Wei and still upholding Jin as legitimate (he reasoned that Jin became legitimate by unifying the empire), became free to write about the Simas' conspiracies against the Caos, since those conspiracies would become righteous acts against a illegitimate dynasty.

Since you also asked

I'm curious if there was some specific reason that the Shu Han might've been the ones who were viewed more favorably over time instead of their more successful rivals.

let's break this down a bit into two separate questions:

1. Was Shu-Han viewed more favourably over time?

Throughout Chinese history, different dynasty's governments as well as many other factions have associated themselves to different ancient personages and kingdoms, not only including the Three Kingdoms but also the Warring States and others. For the rulers and upper roles of government, most dynasties needed to assert one singular lineage of succession through all the previous dynasties, so they especially needed to choose one or another of the Three Kingdoms to be the "right" successor of Han and predecessor of Jin. This doesn't have to mean that they really cared that much about picking a side... even if it was just for the sake of having a consistent list of reign years.

So not necessarily every Emperor or upper bureaucrat cared deeply about whether Cao-Wei or Shu-Han was the more legitimate Han successor. But some people did care, and almost always their opinion of which was legitimate comes from their own circumstances. Some of the early Tang emperors such as Emperor Taizong identified strongly with Cao Cao, unsurprising since they, too, had been military aristocrats under the recently-collapsed Sui Dynasty. Others identified themselves with Kongming's able stewardship (whether or not they felt Liu Bei had a more legitimate claim to succeed Han than Cao Pi), or with other famous personages of the era.

The first really major cultural association comes from the Southern Song dynasty. There is a very obvious parallel between the Southern Song and Shu-Han: the Jurchen invasion of the north had driven the dynasty into the south, and after the Song Emperor and crown prince were captured by the Jurchens a second son of the Emperor re-established the capital in Chengdu while the Jurchens occupied the northern heartland. The drive to retake the northern heartland was encouraged by poets and court advisors using direct comparisons to the Shu-Han situation (even though Sima Guang's history commissioned by the same court earlier in the Song dynasty when they themselves still held the north had used Cao-Wei as the official dynasty of the era). Perhaps the most notable such case was the zizhi tongjian gangmu, written at that time by Zhu Xi, basically a rewrite of the zizhi tongjian which editorialized the historical text into having a pro-Shu-Han slant. So here we see not just particular figures honouring certain past figures, but a more widespread government/cultural association with one of the Three Kingdoms and using that kingdom as a symbol for their own present ideology.

Then the Mongols invaded the Jurchens, and then they conquered the Southern Song, unifying all of China under Mongol rule. Most of the literary history (at least, as far as we know) comes from this era, especially as part of the rise of Yuan theatre. Following from the Southern Song and occupied-Northern Song, and now occupied by the Mongol Yuan Dynasty, the Yuan storytelling tradition likewise always casts Liu Bei and Shu-Han as heroic, and usually casts Cao Cao and Cao-Wei as villainous.

2. Why does Romance of the Three Kingdoms have an overall pro-Shu narrative?

We don't have any direct quotations from Luo Guanzhong or his colleagues concerning his viewpoint on Shu vs Wei, nor any other sort of primary sources of that sort. For that matter, we don't even know for sure that Luo Guanzhong wrote Romance, nor when exactly the first edition of it was written, nor do we have a copy of the original manuscript. If we do indeed wish to believe that Luo Guanzhong wrote the original edition of Romance, there is over 100 years between his death (~1400) and our oldest known complete manuscript (1522), and we don't know how the novel could have changed in that time.

These unknowns in dating and authoring of the novel are problematic for placing the novel in its original context, as well. Was it written during the 1330s or 1340s, and represents a call to action against the Mongol rulers? Was it written in the 1350s, the last years of the Yuan dynasty, as a celebration and assertion of legitimacy for the rebels overthrowing the Yuan? Was it written in the early Ming to reaffirm the new dynasty's legitimacy (especially since the Ming asserted their legitimacy in relation specifically to the Han, rather than the Song), or perhaps even as a cautionary tale (some of the Ming's early court struggles share elements with the late Han's internal struggles...)?

Furthermore, who was the audience of the original text? It might seem strange at first that the Yuan rulers would look fondly upon a novel praising the rebellious southern Shu-Han heroes, but because the Jurchens had associated themselves with Wei, the Mongols/Yuan had in-turn distanced themselves from the Jurchens by associating themselves with Han and Shu-Han, even conducting ceremonies to honour many of the Shu-Han heroes as a method of appropriating these symbols to try and make themselves look more legitimate and calm Han-chinese anti-Mongol fervor.

Thus, we're left with many possibilities. Romance could be pro-Shu because it is part of the anti-Yuan nationalist movement. It could be pro-Shu because of a desire to help establish the legitimacy of the Ming. It could be pro-Shu because that is the viewpoint which Han and Yuan audiences alike identified with. It could be pro-Shu simply because virtually all of the Yuan plays from which it draws much of its themes and content were, themselves, pro-Shu.

Furthermore, if the novel had been written with a pro-Wei narrative, what would this have accomplished, literary-wise, and for what audience would such a narrative be for? The last government to identify with Cao-Wei was the Jurchens, and it is dubious that a Han Chinese author not employed by the Jurchen government would have much reason to write a novel for a Jurchen audience... and besides that, the Jurchen's Jin dynasty had ended a century prior. The Northern Song, two centuries prior.

By the time that Romance was written, be it the Yuan or the Ming dynasty, there wasn't really anyone still espousing a pro-Wei narrative, be it in government pronouncements, theatre, poetry, or otherwise. It could simply be that a pro-Shu narrative was the established, default option and there was no reason to deviate from such.

But I think it is important to highlight that despite the novel being overall pro-Shu and written in the Yuan/early-Ming, it draws from a wide range of sources reaching as far back as the sanguozhi and which are not all pro-Shu. Compared to the highly fictionalized and dramaticized Yuan plays that were the popular Three Kingdoms narrative at the time the novel was written, Romance includes a lot of historical information drawn from older texts even when their inclusion does not benefit the pro-Shu narrative. Wei and Wu generals who are dastardly or cowardly in Yuan plays are given brief heroic treatment in the novel, Liu Bei's advisors protest against some of his choices (in some editions, anyways), and perhaps most importantly the narrative continues beyond the deaths of the most popular heroes to show the failure to restore the Han. Thus, it is also possible to argue that the novel is scarcely pro-Shu at all, that is a rebuttal to the overwhelmingly-pro-Shu stance of the other literature that surrounded it.

Given how little we know of the origin of the novel, and of the presumed author Luo Guanzhong, all of these are simply theories which have not yet been borne out, and there are many more such theories out there as well (e.g. one theory posits that a later Ming author wrote the novel as a protest to the actions of the Yongle Emperor, but put Luo Guanzhong's name on it to give the novel greater fame and/or avoid persecution).

 

As for Mao Lun and Mao Zonggang, some scholars consider the difference between the 1522 edition and the Mao edition to be minor, others say the changes are significant. Once again, there are many possibilities that the Maos' editorial decisions were politically-inspired by the Ming-Qing transition. But, much like the author of the original text, we greatly lack sources describing the Maos' own views on the Manchu rulers. Some have argued the editorial changes made in the 1660s edition are pro-Ming, others argue they are actually pro-Qing, and yet others argue that the Maos made their changes with professional detachment from the political situation. There are strong arguments on all sides, but unless someone finds Mao Zonggang's secret diary we'll once again never know for sure.

For nearly 30 years, I've been playing a video game based on "Luo Guanzhong's" Romance of the Three Kingdoms (Destiny of an Emperor, NES released in North America 1990). I've decided to finally crack down and read the book; I was hoping someone could recommend an English translation.