What you're doing works in theory, but reality can be far different.
In addition to having SEO optimized content and targeting low competition keywords, you may still need to get good niche-related backlinks to rank. Even after doing this, there's no guarantee.
Remember, Google can do whatever it wants and sometimes it doesn't know what it's doing either. There's been plenty of algorithm updates that have caused websites that you would consider to be doing the "right" thing to tank overnight, like when RankBrain came about.
While it's great to focus on SEO and everyone should do it, by no means should it be your only focus.
