I think it might be a mix of write what you know and write what sells to the most people. I know that sucks. And no, I have no idea how people could get pissed off that Rue and Thresh (if I'm remembering his name right?) were black, but I assume its that same people that got pissed off a made up creature like a mermaid could be black.
I talked to someone that said they couldnt even read Hunger Games "because they werent a teenage girl," so sometimes people are just unable to open their brains.
There are some great new fantasy books out there with black MC's or entirely black casts - The Gilded Ones has been a big hit and is set in a version of Africa. For me it was refreshing to dive into a world that had no roots set in the US or Europe.