Blame the people here, not the language. The thing about quicksort is that it is explicitly in-place, something which goes against Haskell's programming model,

Blame the people here, not the language. The thing about quicksort is that it is explicitly in-place, something which goes against Haskell's programming model, so the code (if you use mutable vectors) ends up looking like C. It's not a lot worse than the C version, but it is unidiomatic Haskell, and so is considered bad form; much like an implementation of lazy evaluation or monads would look terrible in C. But you probably wouldn't pick quicksort to begin with. People should just stop using it as an example of Haskell's elegance.