Ok let me hold you right there, King himself stated he wasn't a swordsmen. He was just a man using a sword. Kaku is actually a skilled swordsman (one of the best we've seen in the entire series). Just bc you use the six powers doesn't mean you can't be considered a swordsman.
I wouldn't say he was well written. He had a simple backstory and nothing else much about him.
And as for why he isn't talked about much, I honestly don't know. King is just a cool character with nothing much going for him but his loyalty for Kaido and how bada** he was overall. Yeah he's part of a unique race and has fire abilities and is loyal, but nothing much else is really that special. Now let's bring Katakuri into the conversation and I'll tell you why despite how power cliffed he is, why he seems to be more popular than King (just comparing them).
Unlike King, Katakuri actually has his own personality and own character that isn't just about being a cool antagonist/villain and being loyal to his captain. He actually earned the respect of many fans when he started respecting Luffy during their battle and even stabbed himself in the stomach to show his gentlemen warrior side. He actually has a genuine reason for fighting besides doing it bc he's loyal to his captain which ties with his short backstory and why he started wearing a scarf.
Unlike King who wears a mask only to hide his face bc of his race, Katakuri does it bc of the guilt he felt when his sister got injured bc of the bullies that took their revenge out on Brulee and not him all bc he beat them up for laughing at his mouth. He actually has a bit of development shown at the end of their fight when he fell down on his back and finally admitted defeat and to put down the whole "I gotta be perfect for my siblings" facade.
In short, King is just a cool antagonist. Simple backstory, simple reason for fighting, simple reason for hiding his face, etc etc.