It's a language designer's language--just like jazz is often musician's music :).

Haskell serves as both a vessel and a showcase for PL research. It also has very close ties to a bunch of communities, including many of the people behind Microsoft's languages (VB.NET, C# and F#).

Also, while Haskell does not have a broad presence in industry, it does have a reasonable amount of mindshare. A surprising number of people know and talk about Haskell, even if they've never used it. This also helps influence the design of other languages.