It is a very reasonable choice: your plug-in project has grown to a scale you have never anticipated. Therefore, you built your own system from the ground up, one which you have total control over, instead of keeping to rely on someone else's system as you did before. This would make new features and bug fixes more easy to be implemented.

I am also very curious about what the MKDocs future would be like. Material has been the most popular theme for MkDocs. People are not using it because they have chosen MkDocs, but using MkDocs because they have chosen Material. With Zensical promising (some kind of) MkDocs compatibility, there will be fewer reasons to stay on MkDocs instead of migrating to the new project.