The original Haskell strings were linked lists of characters. This was simple and elegant and worked well with the functional programming approach of the time (1980s, by the way, so maybe Haskell in its origins isn't quite so modern as you think). Nobody was much concerned about high performance string operations in Haskell at the time.

Inevitably, later people wanted to add more performant string types. But should they be lazy or strict? And do you want an abstract representation of Unicode, or do you want something more immediately suitable for arbitrary binary data? Enter four more string types. And now here we are with 5 string types in common use.