Why was Ace's Busoshoku Haki so weak?

[SSJ2]

I don't get it. The world's strongest man's #2 commander could not even use sufficient Haki against an Admiral?

 

[Kyo]

May not have been able to use Haki period, though I think it's absurd that WB hadn't at least taught him a little if anything.

I don't think he was in the New World for that long. Out of universe, Oda goofed up given how prevalent he'd make Haki later on.

 

[Uchiha]

Elemental weakness seems to override a lot of things

 

[sei'taer]

His haki didn't have to be weak, just weaker than who he was fighting. Magma vs Fire has something to do with it too.

 

[Captain Cadaver]

Marco and Vista's Haki wasn't able to cause major damage to Akainu when they attacked him, so it likely had more to do with Ace's actual opponent rather than having poor Haki.

 

[Papasmurf]

Ace was stated to be able to use the Haoshoku Haki, so I doubt his other aspects of the ability were "weak." Akainu probably just has insane stamina and durability, considering even an enraged Whitebeard couldn't put him down for long.