I can't speak for the author, but I do know the reason I personally left it: Haskell-the-language is amazing, Haskell-the-ecosystem is awful.
Like the other commenter I completely agree that knowing Haskell makes you a better programmer, and when I code in other languages I really miss certain parts of it. On the other hand, the entire Haskell ecosystem feels like a half-finished PhD thesis, and often-promoted libraries suffer from fundamental flaws which are "open research questions". There's a constant drive in many Haskellers to make their code as abstract, type-safe, and generic as possible, but unfortunately practicality and ease of use are often forgotten along the way.
It's a great language, but if you're trying to write production code which builds upon the wider ecosystem you're in for a world of pain. In 2024 Rust has probably rifled enough through Haskell's pockets to make it mostly irrelevant as anything but a testbed for programming language researchers.
Like the other commenter I completely agree that knowing Haskell makes you a better programmer, and when I code in other languages I really miss certain parts of it. On the other hand, the entire Haskell ecosystem feels like a half-finished PhD thesis, and often-promoted libraries suffer from fundamental flaws which are "open research questions". There's a constant drive in many Haskellers to make their code as abstract, type-safe, and generic as possible, but unfortunately practicality and ease of use are often forgotten along the way.
It's a great language, but if you're trying to write production code which builds upon the wider ecosystem you're in for a world of pain. In 2024 Rust has probably rifled enough through Haskell's pockets to make it mostly irrelevant as anything but a testbed for programming language researchers.