I created wiki.vim after having used Vimwiki for some years. I wanted to clean up the code and make it less "bulky" - Vimwiki is a filetype plugin, a wiki linker plugin, a plugin for handling/managing todo lists, and much more. There are a lot of very good Vim plugins for e.g. Markdown filetypes and for todo lists. So, I decided I wanted to simplify and make wiki.vim with a primary focus on the wiki specific features: link and url handling and graph features. I've also included the diary/journal features.
It sort of started as a fork. And at the time, the Vimwiki maintenance was quite inactive and issues were not addressed. I tried to fix things, but the code was quite messy.