Simple: The productivity boost is not worth the time investment. Which will give developers a better return on productivity? 200 hours spent learning Haskell

Simple: The productivity boost is not worth the time investment. Which will give developers a better return on productivity? 200 hours spent learning Haskell or 200 hours spent learning more about the business domain they work in? 200 hours spent learning Haskell or 200 hours spent improving their soft skills? 200 hours spent learning Haskell or 200 hours spent networking? Etc.