User blog comment:Ero-cook/Why didn't Ace get to learn Busoshoku Haki?/@comment-3242083-20110704111221

Blackbeard summed it up perfectly: Ace relied to much on his godlike logia powers and payed the price for his overconfidence.

Also consider how young Ace was. According to Rayleigh it takes a lifetime to master Haki, and every high-class user seen so far is at least in his/her 40's, if we don't count Hancock.