The comparison with Closure is really interesting. They make the point that they do less reinvention of Lua than Closure does with Java - functions, standard library, tooling. I'd love to know why. Is it just that Lua has less problems than old-Java
So anything that requires C libs would automatically rule out fennel for a lot of projects that are essentially using someone's lua api as the target platform. Roblox, mud client scripting, openresty, that sort of thing.
And these environments usually have so much added to them, pcre, stdlib extensions, class systems etc fennel works best not making any assumptions about any of that. It's just straight up the lua semantics, and so anywhere lua works it works. I've used it a lot and originally recoiled from this decision but now I think it is genius.
In contrast, Clojure is intended as the language Rich Hickey wanted for writing the sort of applications he wrote, and the JVM happened to be a powerful (and already existing) platform that was suitable for doing that.