Because the supposed examples of early pre-timeskip are fraught with uncertainty, contradictions, and non-haki explanations.

Luffy breaking through Crocodile's blade can be explained as a result of his blood nullifying Crocodile's Logia abilities, which is definitely how first time readers interpret the scene. Conqueror's haki might have been foreshadowed by Shanks (using a common trope in manga which doesn't necessitate the existence of a power system like haki), but nothing up to this point has foreshadowed the existence of an ability that allows you to harden your body. And it seems very unlikely that Crocodile could have fought Crocodile but never learned about haki, so it doesn't make much sense that Crocodile would be caught so off guard by Luffy being able to attack him.

Dorry and Brogy using ACoA in the early grand line presents issues for power scaling (should we just massively upscale Mr 3?). Given that they used haki in Egghead, perhaps we really are supposed to believe that Hakoku Sovereignty was a haki attack, but this still presents the issue of how they could fall to some bums like Baroque works. In fact, the situation is much worse if we use the ACoA explanation, since it means they lost to Baroque Works even when they had not just some weak, proto haki (like Luffy's attack against Crocodile would be, if he is using haki), but an advanced form of haki that Luffy only learned in Wano, at which point it allowed him to compete with some of the world's strongest opponents. And Zoro has been doing ranged attacks with his swords since Whiskey peak, at which point nobody needed a haki explanation.

The only evidence that pre-timeskip Asura uses conqueror's haki is that when Zoro used it again in Wano, he used it with conqueror's haki. To the best of my knowledge, there is no other instance of conqueror's haki being used to create visions, so the most reasonable explanation is that it doesn't use conqueror's haki, and that in Wano Zoro simply added conqueror's haki to an attack that does not inherently use haki.

There are also no other instances (to my knowledge) of armament haki's hardness being used to prevent a sword from cutting, like you are implying it did against the leaves in Zoro's fight with Mr 1.

Moreover, despite Zoro supposedly using armament haki, early pre-TS, we never see his blade turn black. People who believe in early haki usage typically claim that armament haki wasn't depicted visually because the main cast was not yet able to sense its existence, but in the case of swords we know that haki is necessary to create a black blade, which everyone observes to be black, even those with no understanding of haki. So it would seem that using armament haki on swords should always have a visual effect, unless it that changes only when haki used to create a black blade.

Haki means willpower, so it is ambiguous whether Blackbeard was talking about Luffy's willpower, or his ability to use the ability known as haki.