The same reason why Sanji became resembled as a Vinsmoke only after being revealed a such.
The same reason why Ace became resembled as Roger's son after being revealed as such.
The reason is plot right? There was plot required for Sanji or Ace to be unconnected to their pasts and then there was plot required to make Sanji and Ace connected to their pasts. And labor of this plot was actually important. Judge went looking for Sanji because the plot of WCI required it right?
Okay. It's very obvious NONE OF THE PLOT in Wano requires Zoro to be unconnected to his past and none of the plot in Wano requires Zoro to become reconnected with his past. So the only solution is that it's all LOW SCALE. Zoro being unconnected or reconnecting has to be almost insignificant if it is to exist.
Is this what you want? Some half assed footnote in the arc? Some random guy comes up to Zoro and asks if he has birth mark on his stomach and then he says Zoro is his long lost cousin. And you will be satisfied with that?