Here's my theory.
Steve dies sometime in the 60's (didn't he die at one point in the comics back then) leaving Diana heartbroken. But she continues to find solace in her work with the JSA.
Then some event happens (maybe COIE, from the JSA's POV, assuming something like Dataweaver's theory that the E1/E2 crossovers become time-travel instead? Or something else). Diana is killed in battle and reverted back into clay (what actually happened to her at the end of COIE). But she's reborn/Hippolyta recreates her due to a blessing from the Gods. This 'new' Diana doesn't remember her past life, or maybe she remembers it, but not her time in Man's World and her death.
Flashforward decades later. Due to some mystical/timey-wimey phenomenon, Steve Trevor is resurrected/reborn and he crashes on Themyscera again. Maybe this is a ploy by some Gods to get Diana back to Man's World as Wonder Woman? Or a gift? Who knows? (Incidentially I believe this is also something that happened back in the Bronze Age). Diana becomes Wonder Woman 'again' and starts to fall in love with this new Steve, though their relationship is different. At some point, she meets the JSA and maybe they tell her about her past or she starts remembering things. But maybe she doesn't remember the original Steve, and they don't tell her to spare her the pain? And maybe she does remember (or will remember)?
Yeah, it's actually not a lot different from the Kents being alive in some versions of Superman's origin, and dead in others. Or Supergirl being born on an Argo City floating in space vs. being born in Argo City on Krypton before its destruction. The basics of the story are he same, but the circumstances have changed.