HN seems to be very tech community, yet security and things like CTF which do require proficency and experience in both - theory and practice doesn't seem to be popular here
why it may be a thing?
(b) Why do you think it's not popular?
But even without knowing the answers to those questions, in general I've found that anything that requires significant amounts of both theory and practice tends to have a very small potential audience, and HN is no different in that regard.
Those are some of "topics" that challenges/tasks during those competitions do cover: (stolen from dev.to)
Cryptography - Typically involves decrypting or encrypting a piece of data
Steganography - Tasked with finding information hidden in files or images
Binary - Reverse engineering or exploiting a binary file
Web - Exploiting web pages to find the flag
Pwn - Exploiting a server to find the flag
The more detailed description may be here:
https://www.youtube.com/watch?v=8ev9ZX9J45A
Here's ranking of teams:
>https://dev.to/atan/what-is-ctf-and-how-to-get-started-3f04
> On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
What you're looking for is very specific domain. In HN's context 'Hacker' doesn't mean only security domain but The Hacker Attitude (from this - http://www.catb.org/esr/faqs/hacker-howto.html) which could be applied to any domain.
This is truly just a random guess, but maybe it's because CTFs aren't real and real security vulnerabilities are.
It's more a venue for techbro chest-thumping, and being highly opinionated on topics that one only has a surface understanding of.