Why is Bryan Fuller racist? What did he do?
Okay so... (Quite) long story short: He just... He notoriously does not treat his BIPOC, Queer, or women characters well.
Note: This is an important distinction that a lot of people conflate. From what I've read and heard over the last decade, he is great (albeit difficult, but he has STANDARDS for his work and it SHOWS) to work with and treats actors, crew, etc. very well. Many actors & crew actually walk off of projects that he's in charge of when he doesn't come back, like American Gods, because of how much they love his vision for it.
Often when he writes or conceptualizes a character as a woman, BIPOC or Queer, they're treated horribly - and watch out if those identities intersect. They're:
Spoken over and ignored almost constantly
Regularly stolen from (i.e. BIPOC character is doing/thinking it before but then a white character does it and the white character get the recognition)
Disturbingly/disgustingly assaulted or murdered
Villainized even when being the hero
Go through unnecessary additional suffering that doesn't relate to the story while also suffering through the actual storyline shit
Wlw scenes are hypersexual and very psychedelic pornesque while mlm doesn't get the same explicit sexualization (if any)
They're not as robustly developed in background & personality (probably to not be too invested when they're unnecessarily and unceremoniously murdered).
The only one of these identities that he sometimes arguably writes well is for the Queer community, but that's pretty much only white, cisgender gay men (given his identity, that's not exactly surprising). All of this doesn't include the near-daily covert aggressions (a new term for microaggression, since micro minimizes the damaging impact that they can have) that he likely thinks and performs, as many of us do, that are rooted in racism.
He's also said some.... Questionable... things over the years with respect to moral expectations set up by this neocolonialist, neopuritanical society we live in. One thing in particular comes to mind, but I don't want to go through that whole thing again since it causes major fractures wherever it pops up, and I ask that anyone reading this who knows what I'm talking about to not make this about it. If you have more information regarding his treatment of BIPOC individuals, feel free and add to this.