Because he is busy with Dragon Quest XI.
Dragon Quest X just got its third expansion pack announced at Jump Fiesta earlier today, but surprisingly enough, that announcement came with another one- that longtime series director Yuji Horii won’t be directing it.
Now, before you start freaking out and start worrying about the future of the franchise, calm down. Horii confirmed that he won’t be working on the new expansion pack precisely because of the future of the series- he is currently working on Dragon Quest XI, and that is taking up all his time.
That sounds good to me, especially since it’s further proof that Dragon Quest XI is in fact actually a thing and in production. Of course, we still don’t know what platform it’s for- a while ago, the 3DS seemed like a lock, but right now, it’s not so sure. It could be for anything from 3DS to PS3 and PS4 to even Wii U. Or maybe it will be for all of them.
Dragon Quest X, meanwhile, is available for the Wii, Wii U, PC, and Nintendo 3DS.