The HN crowd probably thinks the current meritocracy is good enough, so unions seem like an unneeded expense.

You tend to appreciate it more if it might have direct benefits to you.

I suspect if you asked people who were laid off, or older developers experiencing age discrimination, you would see a different distribution of opinions.

There's also some history in the US of unions lining their own pockets, shady deals, coercing membership, organized crime connections, etc. None of it very recent though.