I'm not really well-versed enough to provide a nuanced take on why one would choose TiddlyWiki over those three, although I think it has something to do with the fact that at the end of the day it's just HTML. OrgMode locks you into emacs, Obsidian (app) and Notion are proprietary. Makes sense there would be a big contingent of systems-oriented people who swear by it.

Anecdotally, one of my buddies uses it to host his homebrew TTRPG ruleset, which the wiki structure actually works very well for. I just download the HTML file and then I can reference it without internet, which is quite nice.