Why didn't Ace get to learn Busoshoku Haki?
You see, he's one of the top commanders of Whitebeard crew.
The other commanders (Marco, Vista, Jozu) know this type of haki. In his 3-year stint (at least) with WB crew, Ace should have known how important Busoshoku Haki is when fighting those with devil fruit abilities.
Or was he just so confident of his powers he felt he has no need of Bososhoku?
With Busoshoku Haki, he could have defeated Blackbeard.