It is a mixture of many things.
Laziness
Chronic pains
And an sick obsession with Dragon Quest.
I dont understand how there aren't any riots from his fans in Japan.
The manga has so much potential but he takes so many hiatus, he said he would immediately start the manga again after finishing the Kurapika special, he did about 7 chapters and then went back on hiatus.
Oh and he doesnt like to have helpers like other mangaka's, so that isn't helping much.
He could atleast write the story and have someone else draw it for him, i believe his wife at some point was helping him, she's the creator of Sailor Moon by the way.