Because PG is a fantastic platform for doing everything you need in SQL and it performs real well. Add PostgREST and you've got a REST API almost for free (you have to design a schema of what to expose, so not entirely free, but still, pretty close). Also, PG moves _fast_ and has a very active developer and user community, which means you'll be getting more awesome functionality in the coming future. (E.g., I've been following the AIO thread and the performance improvements coming from that patch set will be hefty.)