Because to me this is not a matter of capability or strength, but character and fighting style
All Kuzan needs to do to become a Garp level brawler is practice his arm strength
Black blades however are an entirely different matter, you must be an actual swordsman who actually gives a shit about his blade, and would feel ashamed if a single nick scratched its surface. “The will of a swordsman” and all that gibberish, that is what will be the key to black blades.
Ryokugyu is nature incarnate, something larger than life and rivals the power of the ocean. Find it pretty hard imagining someone as grand as this giving two shits about some blade, and trying to hone his swordsmanship to perfection and what not
Fujitora doesn’t lack the haki to make a black blade, only the will.
No top tier cares about swordsmanship, they all have different drive and ambition
Roger wanted to conquer the seas and have an adventure
Xebec wanted to crush the world
Whitebeard wanted a family
Fujitora wants to create a better world
Etc
Only Mihawk has swordsmanship as his primary drive for glory