New article from Variety explained why the first script was dumped and why it was changed.

Leonardo DiCaprio finally weighed in on the massive “Killers of the Flower Moon” rewrite that changed the direction of Martin Scorsese’s latest historical epic. The script originally centered on FBI agent Tom White, who DiCaprio was supposed to play, as he investigates a string of murders among the Osage Nation in the 1920s. That perspective kept the film in line with the majority of David Grann’s 2017 book of the same name, but something about telling “Flower Moon” from the FBI’s point of view did not sit right with DiCaprio or Scorsese.

Source: Variety