For me: because nothing else approaches it in terms of linguistic flexibility. I learned Ruby about two years ago, after learning...a dozen-ish languages? Something like that. I don't write web applications in it, because Rails grinds my gears for a whole host of reasons, but for my money it is
the can-opener language. If I have a problem, I feel like I can generally explore it faster in Ruby (holla, Pry) and build a solution iteratively without really burning a lot of mental effort on it.
The second part, aside from Ruby's general applicability and ease of exploration, is that I think Ruby scales, in terms of application design, from that garbage-y first-cut code to really elegant, really clean code that's fun to work in and easy to maintain. That first solution, borne mostly out of dumping my REPL history, may not be great, but that's okay: with the help of the excellent ecosystem around the language I can reify my expectations in tests and swap out the original exploratory system for good, easy-to-work-with code that's clear in its behaviors. Of course you can do that with other languages, too--but I've never found one that makes it as easy to stretch in both directions, from noodling around to code I'd trust with my business (and I'm doing exactly that), other than Ruby.
But as for the languages you namedropped:
- Perl 6 is great, but it's a decade late and it isn't sufficiently compatible with Perl 5, so it's starting miles and miles behind where Ruby is right now. It's not dead out of the gate, but it keeps lurching toward the green and making everybody nervous. Practically nobody knows Perl 6 now, I don't know why somebody would pick it over Ruby if they're lining both languages up side by side.
- Clojure is super interesting and Rich Hickey is a supergenius and we're all lucky he didn't go into supervillaining because I think he'd be really good at it. And Leiningen is an amazing build system. But Clojure suffers from what I view are the usual problems a Lisp has around readability and the hands-on-keyboard experience (and I like Lisp, I've got DrRacket installed on my laptop for a reason, but I've been writing Lisps for a decade or more now and grinding out Clojure code is slow). Love the people, love the community...would rather write Ruby.
- Rust doesn't do nearly the same thing as Ruby. They're useful for different things. I feel like Rust inherited the good parts of the Ruby ethos from the participation of folks like Steve Klabnik. Big fan of the language and the community, but it's not incompatible with Ruby where it matters anyway so who cares?
- F# is a great language and, like Rust, also has little overlap with what I use Ruby for. If I cared about Ruby for the web, maybe there would be more overlap, but F#-for-web is swimming upstream anyway. (Doable, but harder than C#.)
- Go, while it as a language deserves some small credit for introducing co-routines to people who come from Python (he said, only somewhat snarking while he makes Lua gang signs), is close to an unmitigated disaster of an ecosystem that tries to be prescriptive when it should be hands-off and hands-off when it should be prescriptive. It is also a tremendously limited language in terms of syntactic flexibility and ability to do stuff like "second-order" metaprogramming; this is a design decision that can be debated, or could be debated if I hadn't been convinced long ago by better languages that crippling a programmer to make writing an execution environment easier to always be the wrong thing. Do not want.