Looking back to the '80s...
Let's take January 1984...
DC had 36 comics released that month.
Roughly, about 21 were traditional superhero titles.
The other 15 were:
Fantasy - Amethyst, Arion, Warlord
Western - Arak, Jonah Hex
War - Blackhawk, G.I. Combat, Sgt. Rock
Horror - Swamp Thing
Sci-Fi - Atari Force, Omega Men, Star Trek
Crime - Nathaniel Dusk
Anthologies - Best of DC, New Talent Showcase
I think that the main reason DC doesn't still have the same ratio of superhero:non-superhero today is because with the rise of comic shops as the main (read: sole) distribution of comics in the '90s, indies became more prominent and they mostly focused on non-superhero stuff. And they arguably tend to do genre fiction comics better than DC or Marvel. If only because they're not tied to having to maintain a continuity outside of the comic, itself.
Then combine that with how DC (and Marvel) learned they can make a lot of money with superhero events (especially crossovers) and variant covers. So superheroes got DC's attention. This began roughly with Death of Superman, but the seeds of it really started with Crisis on Infinite Earths and Dark Knight Returns. With a lot of help from Marvel's X-Men showing that an expansive franchise could test the limits of how many comics a reader would follow for one team or group of characters, leading to more and more multi-title franchises.
Before New Mutants, Alpha Flight, X-Factor and Excalibur came along, there were only a few multi-title franchises: Batman (Batman, Detective, Brave and the Bold, World's Finest, Batman Family), Superman (Action Comics, Superman, DC Comics Presents, World's Finest, Superman Family), and Spider-Man (Amazing Spider-Man, Spectacular Spider-Man, Marvel Team-Up, Marvel Tales).
So, while DC and Marvel are producing almost three times more comics than they were in the '80s, there's less variety because a lot of franchises have expanded into multiple titles and, aside from Vertigo and the occasional tester, they both leave non-superhero titles to Image, Dark Horse and Dynamite.
I think with the boom that superhero fiction has been enjoying since the MCU and the CW shows began, it's not going to change anytime soon.
If we do get any genre fiction from DC, it'll be like Justice League Dark: a superhero comic with undertones of another genre. Because they have accepted that their current audience is superhero-specific.
Otherwise, they'll let Vertigo limp along as a tester or just leave the non-superhero fiction to Image, Dark Horse and Dynamite.
At least until the current boom runs its course. Which, it eventually will, just like the Reality Show and the Procedural Law/Forensics Drama booms.
Another way to look at it is that during those times when DC did put out strong non-superhero comics alongside their superhero offerings, those other genres were just hotter than superheroes outside of comics.
Currently, the superhero genre is hot outside of comics, so DC is simply doubling down on the genre since it's the one that they do best with.