Sanderson has been building up a hell of a fanbase for over a decade. Arguably he got a big boost when he finished Wheel of Time, but I imagine he would have ended up more-or-less where he is right now without it.
There's a lot people like about Sanderson, and a lot that people (I think completely unnecessarily) consider controversial. Sanderson is well-known for extremely thought-out magic systems that can be approached as a science, that are often explained in detail in his novels (and then further built upon in future novels). These magic systems share many underlying bones that cross novels; for example, unique magical powers using metal is a common theme in his books, and specific types of metals have common magical attributes across worlds, despite there being different magic systems.
Speaking of interconnectivity, Sanderson is also well-known for the Cosmere, his universe. It has multiple series and stand-alone books in it, some of which are tied in to a gigantic building background event (think Thanos in the MCU), others of which essentially do their own thing. Despite the number of worlds in this universe, each one is surprisingly well fleshed-out, in terms of history, cultures, and magic systems. In addition to the physical universe there's a cognitive one, and a spiritual one, tying everything together on multiple levels. (Most of these books are chunky epic fantasy tomes, although because genre is squishy you can debate that in many areas.)
Sanderson's writing style tends to be very straightforward. He has said on multiple occasions that he intentionally writes to get the prose out of the way so people can just enjoy the characters and the stories. This is a point of minor controversy; some people get weirdly cranky about authors not also trying to be poets, while other people enjoy the simplicity. He has written novels with more stylized prose, which proves he can be flowery/pretty in his language if he wants to, but it isn't a priority.
Another "controversy" are Sanderson's characters. It's no secret that Sanderson writes heroes. That doesn't mean he doesn't have characters with grey areas—or that are genuinely, despicably evil—but this is no Game of Thrones with everyone looking to sleep with, and then backstab, everyone else in the room. For some reason some people take issues with that. (On that note, while Sanderson isn't afraid of implying more mature content, things explicitly stated on-screen tend to be pretty clean, which some fans of more gritty fantasy get weirdly cranky about.)
Sanderson is also extremely unique in how he handles his writing business. Since he first got signed on for traditional publishing, he has slowly been taking most of his process in-house and giving himself direct control, making him a hybrid author rather than a pure traditionally published author. This really unique situation gives him the best of both worlds: he has access to traditional publishing distribution and advertising, but he can self-publish whatever he wants whenever he wants, and because he has such an enormous, passionate audience he's almost guaranteed success. He often traditionally publishes regular editions of his books, then handles all the special editions through self-publishing. He's also branched his business out in other ways, such as starting his own convention, running a lot of merch, and so forth.
Sanderson is also wildly prolific. Despite writing absolute doorstoppers, he sometimes publishes two or three books a year. I am of the opinion that, even more impressive, his quality tends to range from above-average to simply excellent. He does have a few stinkers—in my opinion, most of Era Two Mistborn disappointed—but that doesn't change the fact that he's extremely impressively, positively consistent. (And even is novels that weren't received well still have their dedicated fans.)
Sanderson branches out a bit, writing in other genres and writing non-Cosmere stuff on occasion. Probably his biggest non-Cosmere stuff right now are the Skyward books, which are YA sci-fi that adapts the "boy finds a dragon egg" trope into "girl on a hostile planet finds an abandoned space ship with an advanced AI" flavor. They appear to be pretty successful, but I didn't like them after the first. (I really wish he'd do a Rithmatist sequel. That is probably my favorite non-Cosmere.)
gasps
Oh, did I mention that Sanderson is an adjunct professor? In addition to writing boatloads of novels, running a hybrid traditional/indie publishing house, working with other authors to collaborate in his worlds (a newer thing, most of which hasn't been officially published yet outside of Skyward), and, as noted above, working with other companies to produce more tie-in content (like this TTRPG), he lectures on writing at BYU-Provo during the winter, and teaches an advanced writing course where you get to work more-or-less directly with him and have him professionally edit about 5,000 words of one of your novels. (I took both classes, and it as absolutely transformative for my journey as an author.)
If I really wanted to, I could probably go into detail on a lot more. Sanderson has received multiple film/television authors, but turned them down because he cares about creative control more than money. He wrote the world for a video game a while back (some sort of digital squad-based miniatures battler). He's got a children's picture book coming out soon. Etc.
Sanderson doesn't get the mainstream media attention that writers like GRRM get, which might be why he somehow is wildly successful and well-known while being completely under the radar for many people who see themselves as avid fantasy fans. That doesn't mean that Sanderson doesn't get any media attention, of course, but... I mean, he's just not a controversial figure. The biggest scandal he has is writing four books in secret, publishing them in a single year, and using some of the funds from that Kickstarter to help fund a few other Kickstarters that he overshadowed. A journalist tried to write a hit piece on his a while back, but it just turned into the entire internet (it felt) showing up to defend Sanderson and tell the journalist to go to hell—despite Sanderson explicitly telling his fans to be nice to the mean journalist. And in a media cycle dominated by controversy, who wants to write about the guy that's just consistent, decent, and generally above-average?
