Mercury is probably developed and maintained by a very small, even single-perrson team, at this point, so there's most likely an issue of how to best allocate resources. Perhaps the language designers just can't afford a sleek, web-based documentation system, such as other languages have.
Let's also not forget that, just because someone understands how to design and implement a really cool programming language, doesn't mean they understand how to perform "marketing and good communication". And vice-versa. These are often very different skillsets, like designing a racing car and actually racing it.
Again, it's not a given that a small team will have both those sets of skills readily at hand. Which of course, can stunt its growth, further compounding the problem.
Let's also not forget that, just because someone understands how to design and implement a really cool programming language, doesn't mean they understand how to perform "marketing and good communication". And vice-versa. These are often very different skillsets, like designing a racing car and actually racing it.
Again, it's not a given that a small team will have both those sets of skills readily at hand. Which of course, can stunt its growth, further compounding the problem.