Brandon Sanderson's Mistborn

Hey, all! I came to this sub looking for new authors to read b/c I hadn't read much fantasy in the last few(10) years. After lurking about for a few months I narrowed it down to Sanderson, Abercrombie, Weeks or Rothfuss for my reintroduction to fantasy. Eventually I decided on Sanderson, mainly out of a sense of perversity, because I really hadn't enjoyed what I'd read of his work(WoT).

Anyway, I've recently finished Mistborn, and well, it lived up to my expectations. Great story hampered by poor character development and writing. Which leads to my question:

Why is it that you think that Brandon Sanderson doesn't write YA? I'm very curious.