This question reminds me the news that Noma Dumzweni plays the grown Hermione Granger in Harry Potter and the Cursed Child. Most arguments are about her skin color; many people never seeing her performance claim that they are not going to see the play.

Why people are so conservative about the choice of actor/actress of a role with specific background? Is that a judgement from our common senses, or simply a reaction of the conformity?

I really like the question you raised here, and your defending examples about French and pizza.