zoro didn't fail.
oda got retarded.
the ryuma parallel is something that should be unique to zoro only, but that would require ZKK to happen and zoro be recognize as the second coming of ryuma, so oda out of nowhere took it away from zoro and gave it to luffy, because what would luffy be if he didn't defeat the main antagonist of every damn arc? literally nothing but a mediocre character, so oda must always make luffy have the biggest arc wins just to keep him relevant, even at the expense of plot consistency and other characters development.
oda had said in sbs vol 102 that he writes one piece for "casuals", aka MC-tards, which form the majority of one piece readers, they don't give a fuck about good writing or consistency or build-up/pay-off, they just want to see the MC defeat all the bad guys and have the coolest powerups and highest bounty, and they will be angry if luffy doesn't get all the hype.