Can anyone tell me why my ShowHN submission is not listed?

It is marked as [dead], in your settings, select "showdead" to see this.

Your next question is "why is it dead", I don't know, but you do seem to have a sequence of spammy submissions ...

It needs a 'vouch' from an existing users because currently your karma (points on the top-right of the page) is 0. I vouched now.