I’ll just give my anecdotal opinion. If it matters, here’s some of my rather dull background with respect to my interest in Sanderson’s content. I mainly know of Sanderson from his thoroughly enjoyable Youtube lectures on fantasy writing from the college classes that he teaches and publicly shares. I think he’s a pretty good lecturer. Also, he once gave me some rather useful advice on r/writing when most of the people on there were extremely unhelpful. Having tried to read Warbreaker and found it wanting, I had been curious about this question and when searching Google, I found multiple topics from Reddit and other web forums. I decided to just look-up Sanderson forum posts by actual fans in topics that compared Sanderson to other writers they’ve read. I’ve concluded four major reasons for why Sanderson is so popular, especially on Reddit due to it functionally being oriented towards a hive-mind:
1. His dedicated output of books.
In a time when fans of more popular writers like Rothfuss and GRRM are still waiting over a decade for the next book with fears they’ll pass away before that ever happens; Sanderson’s output is both mind-boggling and refreshing for modern fantasy readers and especially his fans. GRRM and Rothfuss could have some of the best written and most complex storylines, but if they never finish them, then will it matter when there is no resolution or conclusion to any of it? Sanderson serves as a natural contrast for a decade now and fans of his have ever more reason to be cheerful since his shocking revelation of writing even more books as a special surprise to his fans when launching his own Kickstarter to publish and sell his already finished books that he wrote secretly to surprise his fans. When people compare to their disappointment with Martin and Rothfuss still having not released the next installments in over a decade, the contrast obviously puts Sanderson in a well-earned favorable light.
Importantly, Sanderson builds his brand to be authentic. Authentic in his Youtube videos, in the views that he shares to the public, and in writing his books. This is not a situation like James Patterson, who uses Ghost Writers. If Sanderson ever let other writers into his Cosmere, then he’d probably let his adoring fans know it beforehand. Authenticity is a major benefit to his credibility.
3. He has no real competition.
Having decided to simply read Sanderson fans own words, I’ve deduced that they’re a unique subculture of people with certain like-minded views to Sanderson’s own. Judging from their own words and views in their forums: Essentially, socially conscious independent or somewhat right-wing Millennial Christians (possibly mostly US-born and raised) who are very supportive in endorsing Social Justice causes for women, LGBT, and so on; but who personally subscribe to the conservative views and mindsets of their forebears for their own lives. That is, they are perfectly supportive of women’s rights and LGBT people both in their own lives and within their community, but they personally live by socially conservative morals that they grew-up with from their parents’ generation. Apart from being highly supportive of social justice out of what they believe to be a sense of genuine Christian compassion, they personally live by morals of their forebears.
By doing so, the morality systems that they prefer to read are those that don’t make them think of the “sinful world” and which they can think more technically about fantasy magic systems without having to think about moral ambiguity. In short, they prefer clearly defined “good versus evil” to the “messiness of real life” being depicted in their fantasy fiction. They have a lower tolerance threshold for objectionable content. When they try to venture reading stories like GRRM, characters like Joffrey upset them and they likely won’t get too far when characters like Ramsay Bolton are deeply unsettling to them.
Yet, culture changed around the 2010s, where the dominant culture was no longer the dull, garbage of good versus evil narratives, but rather the morally ambiguous with multiple points of view with characters having their own disparate ideas of right and wrong or good and evil. Yet, such narratives make them feel uncomfortable and moving “past” good and evil makes them feel as if the majority of media has largely abandoned them. Yet, they don’t want to seem priggish, because they understand most of these narratives are about people less fortunate than them who suffer from legitimate social discrimination of which these other people were never able to talk about before. They do not want to be their forebears. Yet, they feel abandoned; gone are the days of Tolkien, these are the days of Martin. As a result, Brandon Sanderson is figuratively the second coming of Jesus Christ himself to them.
It’s the real reason Sanderson has such fanfare over this subculture and why the less socially literate of them sometimes inappropriately recommend Sanderson books in subreddits about depression and struggling with thoughts of suicide. Sanderson isn’t just an author; it’s proof that they, too, still have a place in modern culture without needing to change who they fundamentally are. Sanderson means that modern culture has not abandoned them. They have their own place and subculture within the modern cultural landscape. To the best of my knowledge, this is predominately people in the US, but it may also be somewhat relevant to the West more broadly and to some fans outside the Western context due to the more globalized context of the modern world. But again, this is all based only on my own anecdotal observations. This specific subculture are eager Sanderson fans (in some contexts, over-eager), because Sanderson means that the modern world hasn’t abandoned them.
4. Hidden, non-descriptive Erotica under the guise of being subservient to Christian purity culture.
Chapter 7 of Warbreaker made it clear to me that claims of Sanderson being squeamish about sexualization in his books was unfounded. It was merely that, much like the content of his books, they were brief descriptions and not detailed. Although many of his fans will revile and deride this fourth reason; judging from the content of Warbreaker, in which the entire premise is based on a fictional 16-year old having a nude ceremonial bathing and then removing her affluent clothes to submit herself in complete nudity to the King, it is clear to me that Sanderson and his fans merely mask their desire for erotica with non-descriptive terminology in the books and false claims that neither he nor they enjoy erotica. Essentially, they’re reading a sanitized “Christian purity” erotica with lack of description in the events, while trying to enjoy their completely healthy natural sexual desires, but with the unhealthy and unnatural mindset of not doing so because it is sinful per their Christian moral beliefs. In other words, they’re lying to themselves.