Sometimes a thread will get a lot of replies because it strikes a nerve with people - those are often the ones that get closed.

Sometimes the same topic comes up over and over and over, and people end up tired with the topic and feel like they repeat themselves in every thread, so they eventually stop. Which isn't so fun for a new poster who isn't aware of that.

Sometimes people get invested in a user that they've interacted with a lot.

Sometimes people reply because they want to spare newcomers from getting invested in something where the OP just reacts with hostility all the time.

Sometimes someone will reply because they want to mention things from the OP's past threads that seem relevant.

I know not all of that is very positive, or even seems supportive.... but those are some reasons why some threads get a lot of responses.

Usually if a thread doesn't get many replies, it's either posted at an unfortunate time, it's too vague, too convoluted, or people just aren't sure what to say.