Others are doing great covering the actual practical reasons, which inevitably apply to some situations and not others. What applies universally is the psychology part of it, e.g.:
it's a shiny new toy (many of us fell in love with programming because of the joy of learning new concepts)
popularity is a positive feedback loop (gee, I'd better try to do something with FP because everyone seems to be talking about it)
egotistical programmers love explaining arcane concepts to all the plebs who've never heard of them (do you have a few minutes to talk about our Lord and Savior the Maybe Monad?)
those same egotistical programmers have convinced a fair number of people that it's a magical solution to all bugs (there's no side effects! sometimes side effects are accidental and cause bugs! remove all the side effects and there won't be any more bugs!)
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Others are doing great covering the actual practical reasons, which inevitably apply to some situations and not others. What applies universally is the psychology part of it, e.g.: