I think this is a part of the problem that often gets overlooked.
Yes, DC has struggled to make a lot of diverse characters sell. They have, and there's no excuse for that. A serious push for proper representation should have begun no later than the 80's, but its only been in recent years that DC has given this (another) shot. They've tried at various times in the past, but those almost always felt like gimmicks, and I never got the feeling that either DC or the fans ever expected most of those books to succeed. It just felt like going through the motions so they could claim to be champions of progressive social change.
Marvel is just as guilty too. Historically they have been worse at representing women and PoC in solo titles (worse in team books too outside of the X-Men), and while they current dominate that part of the market its a very new thing. They're far from innocent here.
So the companies should be held accountable, but so to should the fans and retailers.
DC and Marvel cannot and will not keep a book on the shelves that does not sell. And they shouldn't have to. Now, yeah, a lot of their diverse books have been pretty bad (a lot of their straight white guy books are pretty bad). But even the ones that are good fail. DC's readership is old and conservative. They dont want new books with new characters, they want the same characters they grew up with 30-40 years ago, doing the same things in the same costumes with the same villains. And for DC, that means white, straight male characters (since their biggest names were created in a less enlightened time). I dont believe its a "race" thing, I think its a "I dont want to try new things" thing. If Superman or Flash had been black from the start, we wouldn't be having this discussion. Alas, the 30-50's was no place for PoC to be leading characters. Hell, back then they were lucky if they were actual characters, and not just bad stereotypes.
DC is to blame for their troubles with diversity, but the fans and retailers deserve some of that blame too. We're the ones who under-order new books because it doesnt have a Bat on the cover and we expect it to fail. We're the ones who complain about a black man being given League status despite the fact that the character has been around for over three decades, held League membership before the reboot, and deserves the attention (even if there were better options). We're the ones who pass up the good diverse books so we can get the seventeenth Batman book that came out this month.
What DC needs to do is completely re-work their marketing and promotional habits. Great diverse books have failed because DC couldn't reach a new audience. If you want to fix diversity problems at DC you need to address this first, otherwise even the best diverse-lead books will fail. Then you need to get new talent who can speak to that new audience and tell stories they'll enjoy. DC tried to attract a new audience with the New52, but what they ended up doing was using every bad 90's troupe out there, which might have appealed to young, potential fans twenty years ago, but not today. It wasn't until Batgirl of Burnside that they finally stumbled upon something that could appeal to today's youth.
Manhunter was an amazing book featuring a female lead. Dial H was a great book featuring a old woman and a fat guy for leads (body type counts as diversity). OMAC was a great book featuring a...crap, what was he? Korean-American? The Others was a decent book featuring a whole team of diverse characters. Vibe was a decent book featuring a Latino lead. Cyborg (Walker's run) was a decent-good book featuring a black lead. They all failed because DC's fanbase is only interested in following the same characters they've been following their whole lives. So to fix that you need a wider fanbase.