Because it isn’t the correct place to execute this. Diversity absolutely matters, but replacing well established actors for characters to achieve this is wrong. For example, Janet Varney has chosen to step down as Korra’s VA after essentially being guilt tripped for portraying a character from a non-White inspired background. She has already become the vocal embodiment of Korra, so it’s too late to change that. Doing so will lead to discrepancies in voice and character portrayal down the line. Plus, this is all voice acting. Your race/ethnicity/looks have no impact on end product. Voice actors should be able to portray any character they like. What’s most important is the skill and talent of the individual and how well they can capture that essence.

Where ethnicity really matters is in real life cinematography. The new live action Avatar remake is being made with appropriate actors from the characters respective inspired ethnicities. They did a fantastic job with the casting. I wouldn’t have it any other way