"The previous generation had too many contenders for WSS for any of them to be definitively be crowned"
Or, get this, or... none of them had the ability to stand out because they simply weren't good enough, which is why they're all black-blade-less.
They couldn't stand out in the "Great Pirate" era
Whereas Mihawk stands at the pinnacle of the Greatest Pirate Era, current day.
And if Prime Mihawk was present in the mere great pirate era, he would be at the throne of the strongest too

Zoro wasn't born too late, he was born in time as per shonen narrative