I'm not really a haskell fan, but the lion's share of that effect more that likely comes from the small sample size of companies using haskell. Even if it were somehow superior, there just are enough people trying it to be coming up with a unicorn startup or two.

Now, the lack of skilled haskell programmers on the other hand, that's a pretty scary proposition if you're starting a company and may find yourself riding on a rocket, needing as many able hands as you can possibly find.