Per the "Win32 event loop" comment, I suspect you're referring to https://bitquabit.com/post/the-more-things-change/ , where the author points out that Flux+React looks an awful lot like a Win32 WndProc function. (HN discussion thread: https://news.ycombinator.com/item?id=10381015 .)
1. It was there at the right time, when people started getting serious with React and were looking for a way to structure their data flow.
2. Tooling. Soon after it was released, the author demonstrated a pretty neat hot-reloading utility.
So no matter whether you're really into functional purity or are reminded of Win32 event loops, it was a practical solution.