There are different applications of Armament Haki. This isn't about level 1, 2 or 3. Luffy's affinity progresses to internal destruction Haki, as it is fitting for his character.
In order for Zoro to be able to acquire internal destruction Haki, that would mean he would need to have the same exact affinity for CoA as Luffy does, which again as I stated is pointless for a Swordsman. Why would you want Zoro to have an advanced form of CoA that is useless for his character? Instead, he should have a different affinity, so whatever advanced form he gains actually benefits him.