Marketing and good communication design is important though, if you want to actually reach new users. If somebody can't get the gist of what you're about from a glance, they are far less likely to invest a year into giving it a solid try.
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.
I tried reading the docs (in the html version), and the first example is in the fourth or fifth page.
https://www.mercurylang.org/documentation/papers/book.pdf
:- module hello.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
main(IOState_in, IOState_out) :-
io.write_string("Hello, World!\n", IOState_in, IOState_out).
Mercury also has HTML documentation, which is the way to go. However it's missing code samples in the home page and it's not like everybody knows about Mercury. Compare to https://www.ruby-lang.org/en/ which instead is pretty well known and still has code to show at the very beginning of the home page.
It doesn't have to be hard evidence exactly, but if you're claiming that something is easy, fast, "modern", what makes it so?
I concede that you can get away with it to some extent by splicing in "intended to be" in the sentence, but it still places on the visitor the burden of figuring out the "why" behind the marketing.
Most devs don't have the time to pick up esoteric hobbyist languages because they're clever.