Zoro's backstory is heavily tied into Ancient Wano, so Oda is saving it for later plot reveals.
Same reason we don't have Kaido or King's full backstories (though it's possible we get Kaido's backstory from King at this point).
Zoro is Ryuma's son sent to the future by Toki. His three gold earrings are a reference to Wano being the "Land of Gold" in the past. The reason he's lost is because he's "lost" in time.
The theme of Wano is time travelling Samurai, and, at this point, the only notable Swordsman from Wano who Zoro COULD be descended from is Ryuma. So... get ready to see salt over this happening.
Same reason we don't have Kaido or King's full backstories (though it's possible we get Kaido's backstory from King at this point).
Zoro is Ryuma's son sent to the future by Toki. His three gold earrings are a reference to Wano being the "Land of Gold" in the past. The reason he's lost is because he's "lost" in time.
The theme of Wano is time travelling Samurai, and, at this point, the only notable Swordsman from Wano who Zoro COULD be descended from is Ryuma. So... get ready to see salt over this happening.