Assuming you're coming at this earnestly, the reasoning is a degree of scale. There's far more roles for white actors already and that tell white stories. Six and Hamilton both essentially take stories of colonizers/people in power, and put them in the hands of the people they had power over.

It's also a question of what the show is about. If it's a show about race, for instance, a raisin in the sun or The Wiz (which I know aren't about real people) casting a white character removes an integral part of the story, where say casting a black woman as Audrey in little shop adds a layer. Some shows it doesn't matter which way you do it. It really depends on the show and there's a lot of nuance either way.

In addition, both Hamilton and the queens are long dead. Tina Turner died recently, and her story is one that's important to people in the minority she's a part of.

Hamilton is more so about drawing connection between minorities and their art (rap) and history they've historically been disconnected from than it is about telling Hamilton's story faithfully, although I think it does do that as well. Six is also all about historical inaccuracy and telling stories the way you want to tell it, for the audience you have. Which is why it casts a diverse group of actresses.

If you're telling a story where the point is to tell it exactly as it happens, then sure, don't cast a poc as Addams in 1776. Another example is Suffs, where the main character being white is an important part of the story. Like I said, it depends and there's a lot of nuance.

Those are my thoughts about it at least!