Mihawk's title places him above anyone with a sword, it is that simple. Nasujuro cannot possibly be stronger, this is just Shanks v2...
However with Nasujuro, the dude is a gorosei, which sets him up to be one of the final villains. Mihawk doesn't have that portrayal. Becoming WSSM is paralleled with becoming Pirate King, which Luffy and Zoro will do before they fight the world government. However this is fine because they each have secondary dreams.
While Luffy's secondary dream is still hidden, Zoro's was stated very early on.
Zoro promised Kuina that his name will be known even in heaven. How would beating Mihawk accomplish this for Zoro? Only by beating a god like Nasujuro can Zoro make his name known in heaven.