I hate that this gets mentioned as often as it does. Because from what I've seen, this isnt on DC. At least not completely. DC has tried. In 2011, DC launched several titles with female and PoC leads. We had several characters running around who were gay (or bi, or what have you). And a lot of those titles were pretty solid books. Some of them were truly great. But where were the sales? Where were the people crying out for diversity and proper representation when DC was actually offering it?
DC did it again with DCYOU. Not only did we have several books with women and PoC, but we had a wide range of tones and sub-genres, as DC attempted (once again) to break into the indie crowd with books like Prez (female protagonist). Where were the sales?
It really does not matter what the movies or tv shows do. Those serve a completely different audience with different wants and it's been proven that movie/tv success does not trickle down into comic book success (from the evidence and studies I've seen anyway, and yes there are one or two exceptions; outliers happen). When a big movie hits, the floppies see a tiny spike for a month or two. Trades see a slightly larger spike for a few months. That's it. It really doesn't amount to much at all. Movie synergy serves an audience that largely does not exist and fails to bring in the potential fans it seeks.
DC tried expanding its lineup with the representation we say we want. They've been trying for years. But the fans are the ones who aren't putting their money where their mouth is. It appears that most people read DC for the familiar, comfortable names like Batman and Superman, not the great new characters like Sideways or Jaime Reyes. I blame DC for a whole lot of things and often question their business sense, but the failure of diversity isn't really one of them. Sure, it could be argued that DC could have tried harder, that they dropped the ball with important minority characters like Static (and gods, did they ever drop the ball with him!) but at best that puts the blame on DC for a small handful of books, not for every book starring or co-starring a minority character that couldn't find the sales it deserved.
EDIT: It's actually not dissimilar to the New Coke phenomenon. Coke was losing market to Pepsi and blind taste tests were leaning Pepsi, so Coca-Cola launched New Coke. And in blind tests, people liked New Coke more than Coke. Yet the public rioted because Coke had changed the classic. DC is the Coke of comics; it's not as fashionable as the competition and it doesn't cater to the younger crowd as easily, but consumers don't seem to want it improved or changed because of the status it holds as "the original."