It's actually one of the few genius tricks that Oda has played to maintain the hype and portrayal around Law while still being a swordsman
By leaving it ungraded, Oda does not put a cap on Law's EoS potential. He can be as strong or stronger than Supreme Grade wielders without technically having one
At the same time, by calling it Cursed and then explaining the concept of what a "Cursed Sword" is in Wano, we now understand Law went through the same process as Zoro with his own blade long ago like the path of swordsmen
Oda never wanted Law to be judged by the Grade of his weapon like most OP fans do for swordsmen, but mainly by his DF growth
I also imagine if it was ranked Great Grade then sword fans would use this to constantly downplay EoS Law