None of them gained notoriety with the blade to the extent Mihawk did, it's that simple.
World renowned before the great era of piracy, Xebec and Roger were assuredly this.
But being known as the WSS in the greatest era of piracy is something else entirely.
Simply being the strongest swordsman out of technicality isn't that important, if you aren't recognised for your overwhelming ability. If the statements about Mihawk relentlessly battling are true, then this is also part of that.
It would mean Mihawk relentlessly fought to an extent that surpassed even xebec. It sounds a lot like ryuumas countless battles. I'd say Mihawk just has more volume and experience, whereas for example events involving roger and xebec involved fate and happening to run into confrontations, instead of actively seeking them in the duelling sense Mihawk did.
We know for example roger went out of his way to destroy militaries and whatnot, but I think this just aided to his portrayal of piracy more than his swordsmanship. But Xebec's swordsmanship is renowned nonetheless. But at this time you still had people like roger roaming the seas, so there was no clear number 1 in the world, and xebec ended up losing anyway.
Also oda just may think it's not important to display this kind of portrayal for this era, which zoro is not even in for example. It's not the greatest era.