Jordan transcribed the prototype into JS for adoption; the SML version of React, however great it might be, would have died in obscurity. The Reason project's biggest goal is to show that OCaml is actually a viable, incremental and familiar-looking choice. We've been promoting this a lot but I guess one blog post and testimonial helps way more.
There's inevitably gonna be a bit of doubt at the viability of Reason at first; but to make an argument from authority: Jordan and the team made ReactJS take off even though everyone hated it at first (browse the first HN/Reddit threads; they weren't rosy). I was there to witness it too. I'd like to say we're rather experienced at handling the social aspect required to make such project take off potentially, at least. Technically speaking, deferring to OCaml is a safe bet.
What it means for Elm/ClojureScript: https://reasonml.github.io/guide/what-and-why#dont-like-reas...
We've explicitly listed Elm & ClojureScript on our Why page as good alternatives to check. I've personally used both. The world is big enough for more of such languages.