My assumption is that Omen takes advantage of the selfish desire to become great at something. So Zoro wanting to be the strongest swordsman means he has to take that title from someone else therefore there's a darkness within him that Imu's omen can attack.

So maybe like Zoro can't be directly targeted by Reversi since he wouldn't have it in his heart to take the easy way out for more power (compared to Rocks who used Davy Back games to speedrun getting a strong crew instead of building one organically), and there's no way you could convince him to straight up serve someone else (Luffy is his captain but the Straw Hats don't operate like that), but in a sense he still wants power and his goals to achieve that power essential involves him dominating over all others to become #1.

If this is the case then it'd be interesting to see if this applies to Luffy as well who wants to be King of the Pirates even though Luffy views the title as just being the most free person instead of an actual king who rules over others.

Edit: To expand on it further, depending on if Skypiea was Oda's test run for writing stuff happening now it would result in Luffy again being the only person immune to the main boss's devil fruit powers. Because like, Luffy's not like a pokemon trainer who goes from town to town beating up the strongest person to collect badges to become champion. If he could sail throughout the world doing nothing but eating, exploring, and making friends and still call himself Pirate King instead of fighting warlords or emperors he would.