If they follow Light's escalation of his "justice" from the source material, a privileged white kid living in Seattle mass killing the USA's prison population which is disproportionally black, and a black super-detective taking him down has to be intentional.
So it makes sense why they wouldn't want the protagonist/antagonist to be Asian in this instance, it doesn't really serve the social commentary. I would also assume they wouldn't want someone black or middle eastern.
Well, that was Yagami Light. This is Light Turner, who is a different character in a different story. You do recognize that it's impossible to accurately convey the source material's story in even a trilogy of movies - and this is only one film. The anime was over 12 hours of content and even that was pared down from the manga.
The film needs Light to be more impulsive to get the plot moving, you aren't going to get 30 minutes of him winning Naomi's trust before the soul-crushing "I am Kira" scene, there simply isn't time for it. Even the Japanese live action films changed Light's look and personality.
Okay then let me ask you this. Why is he still called "Kira"? Kira is the Japanese incorrect pronunciation of killer.