Aside from the DQand back pain responses given earlier, I'm more inclined to believe that he's written himself in a corner. While Togashi seems to have a clear end point in mind, it doesn't seem like he knows how he's getting there. Similar thing likely happened with Yu Yu Hakusho... It just ends. Dragonball ended in a similar way as well. When your characters are basically gods, how do you top that point and make it believable and entertaining?
Combined with the fact that we're probably so, so far away from this endpoint, he's probably a little discouraged from finishing it. For now anyway. Despite only releasing one volume this year, HxH is still amongst the top selling manga franchises around. I wouldn't be surprised if he's milking this for years and years to come.