Haskell is a significant departure from most other languages commonly used by industrial programmers. It's a relatively shallow learning curve from Java to Python to JavaScript, but making the leap to a pure functional language is very difficult. Path-dependence plays a huge part here. This isn't just a matter of "people being afraid of what's different" as the article suggests; there are very rational reasons for a profit-seeking firm to exploit the fact that their developers (and those available to hire) are already relatively proficient at writing procedural code.
Another, more important reason, is that Haskell is too intellectually demanding for most industrial programmers. I consider myself an enthusiast of functional programming, but achieving anything practical using purely functional code remains extremely difficult for me, even though I regularly dabble in it during my free time. The Haskell IRC channel can be helpful, but it's very difficult to square "Haskell is easy enough for anyone to learn" with the inevitable "you are too stupid/impatient/incompetent to use Haskell effectively" taunts you're likely to hear, when you're asking for help to perform a simple task. Many Haskell evangelists don't understand that most developers aren't nearly as smart or dedicated as they are.
I'd be curious to know where most Haskell users believe they lie on the distribution of programming ability. I'd estimate most of them lie at the 99% percentile, and that any of them arguing otherwise are doing so out of modesty. (Note that I'd include dedication and curiosity in with intelligence in this metric.) I believe the most likely explanation for this is that Haskell is a particularly difficult language to use effectively.