In the world of tiling window managers, there are a few well-known names. Xmonad is one of them.
Having a small amount of influence within a rather obscure field ends up translating to basically no importance in practice.
There are many examples of very large and critical software systems implemented in languages like C, C++, Java, C#, Perl, Python, Ruby, and others. We just don't see anything like this when it comes to Haskell, however. The examples that do exist are quite minor in reality, and this will naturally make some people skeptical about Haskell's practicality.
And Haskell predates Python, Ruby, JavaScript, Java and C#. It has had a very long time to become more widely used, yet we just haven't seen this happen. It's not like people don't know about it; many students, academics and practitioners have been exposed to it for years now, and it does get a fair amount of hype for a language that has experienced limited use in practice.
I just don't think it offers enough benefit for the cost involved with learning and using it. If the balance were better, we'd see it being used. But that isn't the case, so it sees minimal usage.