There's no clear reason stated in the manga (figures, he only used it 3 times in the whole series afterall...)
But if I had to guess, its probably because it's a last resort attack that requires too much stamina to do (3 sword style + making physical illusion of 4 more arms and 2 more heads)
also another point I might add, if you notice all the times Zoro used Asura, he always did it on land, not in midair, this could imply it also takes a toll on his mobility as well (carrying 9 swords would be heavier than 3 right...?)
and during the fight with King, he was in midair, so I don't think he could have activated it then and there, and just went with a new flying slash attack instead (Dragon Damnation)
but tbh in reality its probably just Oda not wanting to reuse the same attack twice in the same arc, it wouldnt generate enough hype compared to a new attack.