Or, they don't have the talent to create popular characters. There's a reason Marvel/DC mascots are 50+ years old.
Considering the level of talent working on these books, I'm calling bullshit on that.
The truth is the difficulty Marvel and DC currently have launching new is the result of a couple factors. Chief among them being that there just aren't that many people comic books anymore. Which is, of course, their fault because of how they choked the life out of the market with all their bullshit.
Either way, when you have a limited that's more or less set in its ways as far as what it will buy, something brand new has little room to breathe and gather an audience. And the audiences that are open to new and different are forgoing Marvel/DC entirely for Image et al or they're buying the same heroes they know and getting their new elsewhere.
Like I said, money talks, and when publishers see things like "The Movement" fail, see giving established characters like Katana books fail, see even popular characters like Black Panther struggle time and again to keep a book on shelves, they aren't likely to keep sending those books out to die. Especially when they know they can launch a new Superman or Spider-Man monthly and watch the money roll in. They have to go with what audiences respond to.
And when comes to diversity, what have the audience who actually will go out and buy a comic book responded to in recent years? Black Spider-Man. Arab American Ms. Marvel. Latino Blue Beetle. Lesbian Batwoman. Time will tell for the new Thor and Cap, but it's clear that for all the gnashing of teeth online, audiences have shown legacies (and sidekicks; new characters like Harper Row have a far better chance sticking around as a second tier Bat character than they'd have starting out on their own) is what they'll show up for.
Is it ideal? No. Ideally, we could have new characters, established characters getting more shine, and new takes on old favorites. Do I think Marvel and DC do enough? Nope. More minority creators and more of this type of press for wholly new characters would be fantastic for a start. Am I blaming customers? Despite my tone, that's not my intent. I'm simply stating the reality of things as I see them; I believe people can buy what they want.
But to say that Marvel/DC simply don't try at all or lack the creativity to do so is patently false from where I'm standing.