My take.
Believe it or not, I don't think the landing page for fastify makes a compelling value proposition. You would think that "fast" sells, and it appeals to me, but when I have test marketed "fast" I've found people don't respond the same way I do.
At the bottom of the page I see a whole bunch of logos and names that look like the logos and names of trendy modern companies except I don't recognize the names. It just doesn't connect to me.
There are so many web servers and frameworks out there that it's not easy to stand out, particularly in the Javascript world. As excited as people are about Node, and as much as using the same language on the front end and the back end is a "holy grail", most of the people I know who write front ends based on React and that kinda technology still use Java, Scala, Python, Ruby or something else on the back end.
Other than that, HN is not really a pure "technical" forum. I think it uniquely covers people who are interested in both technology and business so if want to get people's attention you have to show people something they haven't seen before and that they recognize they haven't seen before.