The Harry Potter franchise is the biggest in the world, there are fans from every country and even the Fantastic Beasts textbook features creatures from many different world cultures. The brand is so powerful that it wouldn’t have been taking a big risk casting PoC leads in the first film.
It feels like a lack of imagination on the parts of the producers. Only a lack of imagination imagines a magical world filled with only white characters. Only a lack of imagination imagines a world in which a PoC lead doesn’t bring in the same (or better) box office numbers as a white lead (see: everyone’s assertion that Idris Elba as James Bond would break the box office and bring new life to the series). It would have been a wonderful statement and a show of support for PoC fans of the series.
It’s a shame that PoC have to wait to be included.