Donquixote pirates also fight together, but we saw some lower ranking member having haki. WB was shown to have haki, WB was man comparable with Pirate King. So basiclly to be Pirate King you would need to learn haki. Ace was trained to be next PK.From 1-3, Ace might not have needed Haki to fight and survive in the New World because he was a member of Whitebeard's ship. They fought together. If they attacked someone or were attacked, Ace merely fought someone that he didn't need Haki for and left someone like Marco or Jozu to fight people that you need Haki for. Ace survived because he had WB pirates backing him up. Remember when Buggy's crew wanted to kill Ace and Buggy said no because killing Ace meant facing the wrath of Whitebeard? Anyone who could kill Ace in the New World, who were worth their salt in other words, would definitely know about Whitebeard. Ace survived because he had the Whitebeard Pirates in his corner.
Ace was protected by WB, but WB was old and probably aware that he will soon die. Who will then protect Ace and his crew? It would be natural, for WB to teach Ace (and other members) haki, since one day they will no longer be under his protection.
When Logia vs Logia happen, user could just use their fruit powers, instead of haki.1. Ace stalemated with a Logia because he couldn't harm his physical body
2. He has no qualms about potentially killing his enemies
3. He hit Teach, a man he was trying to kill, with an attack not laced with Haki, right after he was told Teach is a Logia
4. Even before coming up with Buso: Koka, Oda had more than 12 people exhibit Haki prior to the timeskip, even minor characters, but didn't depict Ace, not once, using Buso: Koka, and Ace is a significant character
Or he didn't bother to use it, since he himself is logia, and all his opponents were logia (Smoker, BB, Aokiji, Akainu).When you lump all of these facts together, you get that Ace never used Busoshoku because he either couldn't, or he's stupid, and Ace isn't stupid.