https://d23.com/ask-dave-011514-021814/
Skip to question 11 and they explain why. The producers were looking for a retro voice to match the style of the Mickey Mouse shorts, a voice which Iwan couldn't replicate. Diamantopoulous has never voiced Mickey in anything outside of those shorts, so I think it's safe to assume that Iwan will continue to voice Mickey throughout the KH series.
Also, in response to the above question regarding whether Ohtsuka and/or Nimoy had recorded any voicework, I imagine Ohtsuka may have done some voicework, but seeing as how localization, and by extension voice acting for the localization is one of the last things that they do, there's no way Nimoy or any of the other English voice actors had recorded anything yet.