It's a similar thing with rokushiki. There is no reason for Zoro to not be able to do geppou or Sanji to know ryankyaku. It would cover their weaknesses, and they clearly don't lack the strength or talent to be able to do it. Oda just doesn't want to homogenize the fillerhats, which is just hilarious considering haki is the most homogenizing power system in all of manga.