Because at first glance, this is visually only a tiny fraction away from

 

   (if c then f else g)  it d them a elsa b

which would be parsed in a totally different way. (Personally, I think that if/then/else is useless in Haskell and just a concession for readers from other programming languages. Having a plain old "if" function would have done the job in a more consistent way.) Of course syntax highlighting improves readability here, but code should be easily digestible in black and white, too. Visual clues matter...