Diversity isnt just black people. DCEU seems plenty diverse so far between Justice League and Suicide Squad.
SS has multiple PoC and multiple women (Deadshot, Harley, Waller, Rope Guy, Katana, Enchantress, and El Diablo [technically Croc as well, but ehh...I'll leave him off]). JL has two men of color in Cyborg and Aquaman, and potentially two women if my hunch about Mera being the film League's 7th member pans out. Wonder Woman will also portray the Amazons as multi racial.
So yeah. Plenty diversity. Now if you want to say they need more women and PoC in starring roles, I will agree. Though Marvel is slacking here as well. Black Panther will be the first - and for the foreseeable future, only - film starring a man of color in the titular role and Capt Marvel is still a long ways off.
Why would WB need Black Heros?
Because there are plenty of talented black actors who deserve a shot to break out and be stars in what is currently the biggest genre of action film. The whole "they should just go based on talent" argument is 1. insulting, given how many talented black actors there are out there, and 2. naive, given how it ignores how often those very actors are looked over so that Hollywood can cast another white guy named Chris regardless of their actual talent in comparison.
And it's not like DC doesn't have black heroes they could use in films. Mr. Terrific, Vixen, Black Lightning (and Thunder/Lightning), Power Girl, John Stewart, Steel, Aqualad, Bronze Tiger, Bumblebee. All could either carry their own movies or make appearances in team movies.
And this is to say nothing of the Asian, Latino, Middle Eastern, etc characters they could be better utilizing.