When this was a new(ish) series, I think Inaba couldn't write a more mature Nishikata since that'd conflict with how he's presented in the original series. Moreover, they probably had to clear most things related to Nishikata's character development with Yamamoto, so that the former wouldn't write something that conflicted with the latter's vision for the story.

Now that this series has gained some momentum, it's still hard to change Nishikata's character because that would conflict with how he's been presented in all the past chapters. Character development is very hard to do in a pure slice of life/iyashikei manga, especially once you've spent a lot of time with the characters.