Because, you failed to properly set up GHC, you don't have the right to say anything about Haskell or functional programming.
And I'm sure it's possible to tune GHC. The argument is that if you're going to write immutable data structures in ruby then there is going to be more GC pressure. I don't think it's possible to optimise the ruby GC anywhere near the GHC one because of the mutable nature of the language.