Tetsuya Nomura is known for taking his time when making games, most recently with Final Fantasy XV, which was announced 2006 and released a decade later after directing responsibilities were handed to Hajime Tabata in 2014. He has also been working on Kingdom Hearts III for a very, very long time, but Nomura doesn't think he is the one to blame for this.
Instead, he said to Gamespot that there have been decisions made above his head among senior management at Square Enix that has lead to this, most notably a change of engine.
"A lot of people have been mentioning and making it sound like 'Oh, Nomura's taking too much time.' There was a decision made to change to an external [engine]...Unreal Engine 4... So we switched over to that. Unfortunately, there was a bit of time that needed to be rewinded and started over. So there was a bit of a setback there, but it was a decision that the company had to make, so it was inevitable."
Kingdom Hearts III still has no release date other than 2018, but we recently got to see more of it when it was revealed that Toy Story will be featured prominently.