Hi. My startup was listed in IndieHackers, and here is my experience, which I think might explain why this thread did as well as it did. A few weeks ago, Courtland, the dev who built IndieHackers contacted my via email with a polite request for me to list my startup on his upcoming site.
He had obviously done a lot of work scouring HN and Reddit threads to pick out people who fit his criteria of starting independent apps and emailed them one by one.
I was interested, so signed up on his site and started filling in my profile. And here is the clincher - I've filled out my startup profile on LOTS of sites, but IH was the only one where the creator emailed me back and asked for more information and clarification about my story. It felt more like an interview than a straight up submission, and it also lead me to believe that someone actually cares.
Then, yesterday, I received an email from Courtland mentioning that he would be posting about his site to HN as a "Show HN" post and asked me to support it by upvoting.
By this stage, I had totally 'bought in' to the whole deal, and when I woke up this morning, eagerly sought out the thread to participate and upvote. I was actually alarmed when I couldn't find the thread. A quick email and response from Courtland indicated that the thread name had been changed from a 'Show HN' thread.
I am thinking that it was the personalised contact with the participants, then the email follow and actual request to support the thread which lead to it getting the response that it has to date.