Rebol itself is practically dead, no new release in a decade. It was also proprietary early on and a "weird" language (interesting, and good, but non-standard in so many ways that it was very niche). That killed its potential in the late 90s and early 00s when everyone was moving to open source scripting languages that filled the same niche, but with more conventional syntaxes and free access.

That's why it isn't more popular. In some ways it was a few years too early (to hit the zeitgeist around DSLs), but it was also a few years too late for its proprietary model.