Blame oda for going out of his way to say king isn’t a swordsman despite using a sword after throwing a punch, specifically saying mihawk’s sword skill is better without saying he’s actually overall stronger, for giving him a twin brother with a mythical devil fruit sword and giving shanks a sword also named after a mythical creature, giving shanks significantly better portrayal hype and feats etc
I changed kings name to shanks to make it easier for you to get
Being able to cut iron is an important step taught to all wano swordsmen and as far as I know kaku can’t do that and barely even uses his sword or any sword style techniques
If it’s a mythical creature like his brothers it opens up an arm
Same reason king, queen, big mom, kizaru, arguably Rayleigh, etc skill punch and kick despite being able to use swords