There's no reason to do a black Hal when you can just do John Stewart, a vital, rich character with tons of storytelling opportunities and born of the need for more black heroes in his medium. Just making Hal black instead of having John star is part of the problem. It's like saying "why adapt these works by and about black people when we can just take stories created by white people about white people and make them black?"
Yes, you can still do that, but you're essentially still saying "all the black source material isn't as juicy as the white stuff and we can modify it." Characters like John Stewart, Black Lightning, Vixen, Static and so many others will never get a shot if AT&T can just racebend Superman, Batman and Wonder Woman and call it a day. Calvin Ellis is something movie going audiences have never seen before and offers everything Clark does and more for what they're looking to accomplish. It's just the more interesting take. Hell, he offers them a little freedom because nobody can say "Clark wouldn't do that," much like the arguments made for why DC wants Jon to take over-- similar Superman feel without all the hangups of 80 years of cultural familiarity. Across every avenue, he just affords the project more creative liberty and is a better fit for what they want to do.
I know I'm personally very excited for Infinite Frontier for Alan (a now out-of-the-closet gay man) and Calvin (a black Superman) leading the narrative drawn by Xermanico and written by Williamson (who I'm not super keen on, but I'm sold from what comes earlier).