TL;DR: Because the Postgres folks have outstanding taste, even better documentation, a good community, and a compatible license. Which are all the same reasons

TL;DR: Because the Postgres folks have outstanding taste, even better documentation, a good community, and a compatible license. Which are all the same reasons that it's my goto database. I don't build software anymore, but the last project I worked on for money was a query engine, and the project founders were pretty open about the fact that whenever they were unclear about the behavior required by the SQL spec, they looked at the PostgreSQL docs and behavior to get pointed in the right direction.