Postgres was developed over a long period of time with careful attention to architecture, robustness, and extensibility. It also focused on both external and internal documentation and cleanliness. And, it followed the traditional database system model with a cost-based optimizer and everything else.
As a result, we see a very robust developer community. It's a real machine -- churning out high quality releases every year with a great mix of newsworthy features. Enterprises and startups alike are flocking to postgres for a huge variety of use cases.
Slow and steady wins the race (and it doesn't hurt to be 5 tons and have tusks) and isn't tired afterward. And in databases, it's good to play for the long term because that's how long the data will last.
I wouldn't trade that for some first-to-market popularity.