According to this petition, which I will post below, it is because.....
-Warning this is how I am interpreting the petition, not its exact words-
Instead of coming up with original black ideas, taking inspiration from other ethnic folklore stories they decided to make a black girl play a white character in the name of inclusion. This is not how inclusion works. People of color want you to represent them. This shows that Disney does not really care about other racial ethnicities.
They only want to look inclusive and that's the tea. The little mermaid is a fairy tale that belongs to Nordic (especially Danish) folklore. You can't make European characters black in the name of inclusion! It's so forced and disrespectful
Recast The Little Mermaid: chng.it/ZLjGQYRTns