You don't see a White People Museum because, in the US and Europe anyway, that is the default: they are all white people museums unless otherwise stated.
And, okay, to really shut this one down, what would you say to a "White History Museum" in Africa? That's a real exception to your hypothesis!
Who has the authority to establish that threshold? Seems quite arbitrary and problematic. White History Museum in South Africa? What about Zambia? DRC? All have a white historical presence, and in places like Zimbabwe, an argument could be made that the culture has suffered a certain persecution (I am not taking a side on this argument here).