He wasn't, until a grieving widow handpicked him to finish a highly successful, wildly popular story that was on the verge of its long-awaited climax. WoT is not like other long series I've read, like aSoI&F or MBotF, where you wondered where you were going as you read - the epic, climactic confrontation between good and evil in the person of the main character. WoT was not looking to be FINISHED, it needed to be COMPLETED. Sanderson was handed a captive audience, and judging by the traffic on this board, has driven a considerable number away. The long hiatus between KoD & tGS, even with the death of RJ was not so devoid of interest and speculation as the last few months have been.

None of which Brandon Sanderson did. Actually, from the few chapters I have been able to plow through in Mistborn, those are his strengths. Instead, this project has absolutely no need of those strengths. What it requires is attention to detail, understanding of characters and themes, and the talent to capture the voice and mentality of the characters. The fact that Sanderson has multiple writing projects going at the same time (according Wikipedia, he has had 13 non-WoT books published since he was announced as the replacement author) suggests that his capacity to devote himself to the necessary details is already compromised.

Luck of the draw in getting a publisher's favor? Note that he initially succeeded as a writer of kids books. That does not require a lot of the qualities we notice a deficiency of in his WoT books.

Even successful writers don't always know what will work and what won't. John Ringo got a six book series out of a personal cathartic writing project he thought was crap and is plainly embarassed about. He had apparently mentioned it in passing, had some interest, began showing it to people, and eventually his publisher urged him to do it. He wanted to publish it under a different name and label, but it became the longest-running book series to his credit. His arguably most famous series is stalled at the fifth book with a battle having just begun, while book 6 of his 'crap series' is due in January, despite the prior book being an acceptable end point of the series.

The TV writer and showrunner Shawn Ryan's most successful and critically acclaimed series, The Shield, was originally created as a test or writing exercise. Of the four or five shows Ryan created, none has come close to "The Shield" in longevity or commercial or critical success.

Even on a microscopic scale, I myself have experienced this with posts on the MBs. Things I thought would be funny or interesting, were all but ignored, while throwaway gags have had lots of positive feedback and even appeared reposted on other sites, or stirred up intense discussions. Things I worried about posting because I thought would be controversial or offend people's sensibilities turned out to be well-received, even by people I thought mature enough to know better. There really doesn't seem any way to tell in advance how or if something will succeed, and publishers are probably no better at guessing than the rest of us, so the get-published game is probably not the meritocracy aspiring writers like to imagine it is.