I think it's true that Haskell is better for playing around with the stuff published in ICFP and related conferences. The reason, however, is a bit more subtle. Haskell is the dominant language for programming language research. So publications tend to use it, and Haskell is better adapted at being Haskell than Scala :) There is also more stuff being published in Haskell for this reason.
However it is not correct to say that Haskell is more expressive than Scala. It is in some ways but Scala is more expressive in others. There is a paper that discusses type classes in Scala and show they have some advantages over Haskell's implementation. As more people use Scala and discover neat things you do in it, this is feeding back into other languages. For example, here's the mighty Oleg implementing the cake pattern from Scala in Haskell: http://okmij.org/ftp/Haskell/ScalaCake.hs
Thank you for the link. The cake example was exactly what I was thinking of when I said that things may be changing (though I couldn't recall the specific example). It's also quite likely that as the Scala community grows, and people stop trying to make it be "like <language> but more <quality>", we will see more innovation in this space.
Still, I think Scala is first and foremost a practical language. Straying too far from that as a primary goal would probably be harmful in the long run (i.e. driving a Ferrari might be fun, but you're not going to buy one for the daily commute).
However it is not correct to say that Haskell is more expressive than Scala. It is in some ways but Scala is more expressive in others. There is a paper that discusses type classes in Scala and show they have some advantages over Haskell's implementation. As more people use Scala and discover neat things you do in it, this is feeding back into other languages. For example, here's the mighty Oleg implementing the cake pattern from Scala in Haskell: http://okmij.org/ftp/Haskell/ScalaCake.hs