Basic story telling really.
Zoro's series long journey has been to become the WSS. The fight where he becomes the WSS is naturally then, his greatest achievement. It would be extremely odd for Zoro to go into his final fight already the WSS, because then what are the stakes? We already know he's going to win, he's the WSS. A defence of the title is dull, it's getting the title itself that's dramatic.
If Zoro has already beaten Mihawk... why should we care about fighting Shiryu? It would be like if say Sanji had beaten an Admiral prior to the final war and then was going to fight Avalo Pizarro. That's not an interesting fight. But if it's Pizarro who's beaten an Admiral prior to the final war, now that's interesting.
It's all about stakes. The final fight has to be the greatest challenge. That has two requirements- personal strength of the opponent seemingly greater than that of our heroes, and horrible, world-altering circumstances if the Strawhats lose. Mihawk has the strength, but not the consequences. A final villain needs both.