Why is Haskell so popular?
Lisp has been around for much longer, and is the same paradigm. So why has Haskell captured all the attention in recent years?
Along with convergence of languages and tools, there is also a recognition that isolation of side effects and composition of operations are the fundamental mechanisms for building large software systems from reusable components, and Category Theory is the formalism for describing composition. For example, in the Haskell community there is an ongoing exploration of the compositional properties of isolation primitives like Monads, Arrows, Applicative Functors and Comonads (yeah, Category Theory terminology is a barrier to progress). Haskell is not the final language, but it is the medium in which the correct path is being discovered. I would also say the move from Java to Scala is also a step in the right direction of more powerful type systems and a (more) functional approach.
On the Lisp side, there has always been a cacophony of variations, and people might have had high hopes for Arc, but the future is Clojure.
Interesting that the two most innovative recent languages, Scala and Clojure, have been built on the JVM.
The languages support different means of abstraction. e.g., Lisp has macros, Haskell has all that type inference enables. I can't honestly say that being able to (effectively) use the return type of a function as a parameter to that function is better than being able to invoke macros with the same syntax as a function call --- but it suits me better.
Readability counts!