Very good question; here are my guesses:
We are exiting a Dark Age when the use of non-mainstream languages was nearly impossible (I suspect this has to do with the dot com bust: companies just don't have the money they used to have to throw at "safe" mainstream choices).
I strongly suspect PG's success story and thorough explanations of why Lisp wins made more people willing to give Lisps a chance.
The more varied syntax (e.g. heavy and appropriate use of vectors which are denoted by square brackets) finds favor with a lot of people who think LISP stands for Lots of Irritating Sets of Parenthesis.
The multi-thread/multi-core "future" is here today and many have come to the conclusion that functional programming seems to be the best approach we can see today (there's been a lot of research into this since the '80s or earlier, nothing else has really stuck to the wall; note that Microsoft's F# dialect of OCaml is perhaps the first "mainstream" functional language). Lisps have always had a good story there as the first widely used functional language (well, technically most of them are multiparadigm but strongly functional), with a strong mathematical basis (the lambda calculus). Clojure takes full advantage of this.
Apropos of the above, Haskell, the ML family and maybe Erlang were probably the only more popular strongly functional languages prior to Clojure, but for many they are harder to master; Lisps are more straightforward, they are dynamically typed, etc.
Basing it on the JVM makes a big difference: for some it provides a warm and fuzzy since Java is so widely accepted and the Java/JVM ecosystem is so big and healthy. For many this provides an easier path to getting started, and if you want to get real world stuff done quickly, there's all those Java libraries and packages that can easily be directly called from Clojure and that can call Clojure.
(Actually, running on the JVM makes it easy for pretty much everyone to get started on most any platform, just download the .jar file and run it in the JVM that most likely is already installed on your system.)
Rich Hickey has made a lot of good choices in designing the language and its core libraries; it "feels" like a good fusion of Scheme and Common Lisp, richer than the former and not at all crufty or impossibly massive like the latter.
The community is very functional, so to speak ^_^. There's one Google Group where most of the action happens, it's very friendly and helpful and nasty people are not tolerated on it.