Is there any recent example here of launch of a new site that went on to do well in the SERPS? It would be great to read their detailed story on how they went about it.
Google has massively increased the number of changes they introduce for the past couple of years. Considering this, I don't think it would be that easy to tell the correct reasons for the OP's question, though he has given a detailed account of his new launch. People are only going to guess their reasons here, as several known and unknown factors are at play. Even the popularity of people behind the site/startup is a success factor these days.
Moreover, I think any meaningful study of SERP rankings can be done only after a min. period of 6 months to 1 year from launch. So if I were you, I will only focus on studying users on the site and focus on improving their experience.