The other thread was closed so..
He should've been doing that like 5 years ago.
He didn't want to. He actually WANTED to tell the stories of BBS, Days, DDD and, yes, even Coded, before moving on to KHIII. He has said that each of those games serve a purpose in telling about some period of time in certain characters' lives. BBS is the Origins story, Days shows more of the Organization XIII operations & what exactly happened with Roxas during the year Sora was asleep, Coded's story shows what happened after KHII's main story that prompted Mickey to send that surprising message in the bottle to Sora at the end of KHII (yeah, they really didn't need a whole game for that, but they did it anyway) and Dream Drop Distance is the set-up/preparation game that leads straight into KHIII.
And 5 years ago he was still of the opinion that the OG KH Team that is working on FFXV would develop KHIII once FFXV is done. Since then, things have changed. FFXV was moved to next-gen (and thus delayed finishing FFXV's development even further away into the future) and Osaka Team has proved to be a very competent team at handling Kingdom Hearts. The latter part means that Nomura doesn't consider the OG KH Team as the (active) main KH Team anymore, but has given that honor to the Osaka Team and, with that, has fully trusted the development of KHIII to them. 5 years ago he wouldn't have done that, but would have insisted he'd need to finish FFXV so that the OG KH Team would be free to develop KHIII.