Meta reason is because of Zoro.
Oda decided to give Zoro swords that have been wielded by the two heroes of Wano, Oden and Ryuma.
He also doesn't want Zoro to simply receive a supreme sword, he wants Zoro to make his own.
So that meant that neither Ryuma nor Oden could have a supreme grade. That also means, by extension, no one else from Wano could have one because it would make them look better than Ryuma and Oden. No one in the present day story, anyway. When we get the Void Century flashback we could well see the progenitor Kozuki with one
In-series Wano probably forged the majority of meito, including the supreme blades, and they've just lost them over time to outsiders. We already know Kitetsu made one and he was from Wano.