@Wildfire Kaios
Here is really brief summary which I can make a proper post on Zoro's evolution as a character pre time skip
-->He started out as an arrogant frog who thought he was on the World only because he was a hot shot in East Blue.
-->He meets Mihawk and Mihawk makes him realize he is but a frog in the Well. He gets his assed handed and makes a promise "I will never lose again". Since then his entire demeanor changed..he laughed a lot less..he was like he has a chip on his shoulder.
-->At Alabasta his approach to swordsmanship evolved when he said his true meaning as a swordsman should have been able to cut everything he wants to cut and nothing he doesn't want to cut. He wants to cut to protect who he needs to protect
-->At Skypea he showed another layer of him not caring about those who call themselves as God. It is kinda similar to D's being enemies of Gods
-->Water 7 was all about him showing leadership, with Chopper in Davy Back fight, with Nami and Luffy on roof of Water 7, Usopp Speech
I mean there is a lot more.
On top of the fact that Zoro's Goal and Trigger for that goal is well defined..his motivations are well defined.
To me that is a lot more compelling than Luffy whose character seems mumbo jumbo "I WANNA BE FREEE" like why??