Then why advertise that? Why not advertise their benefits instead (fast, cross-platform, easy install)? "Written in X" is only useful to advertise pedagogy or integration.

Shorthand. Compare the lengths of these two strings:

    > fast, cross-platform, easy install

    > Written in Rust

Also note how the second statement expresses a single concept; whereas the first statement is a list of concepts that must be memorized and reproduced by rote, and is therefore constantly at-risk of being stated incompletely or inaccurately.

(And it is incomplete: there are many more reasons why "Written in Rust" might be important, such as correctness, security, and ease of maintenance/refactoring.)

* * * * *

Alternatively: why convey a set of vague descriptors when you can instead convey precisely why they are applicable, and let the audience's pre-existing memories fill in the details for them?

Like I said, "written in X" conveys pedagogy and integration. (Or, if we're being cynical, hip language clickbait.) It's not a very good shorthand.

Because it's a post for hackernews, and we're mostly engineers and developers so we care to know what it's written in. Rust is a awesome new language with growing popularity and it gets hits whenever it's posted.