Earlier quoted context omitted.
Because GIMP has a lot of baggage: a very particular UI, a community of active users (many of whom probably like it exactly how it is thanks), and a terrible name.
So fork it. New name, new UI. Seashore was a try at this a decade ago. http://seashore.sourceforge.net/The_Seashore_Project/Screens...
A lot of coders doing side projects do it for fun and to learn and that often means making their own architecture decisions (and mistakes) so they can practice that process and learn