- Postgres is probably already running (it's pretty good for OLTP workloads)
- Operational ease and robustness
- Cloud support everywhere for Postgres
- People know how to backup and restore postgres
- Sometimes Postgres will beat or wholly subsume your specialized system and be a good choice
- Postgres has ACID compliance and a very good production-ready grasp on the research level problems involved in transactions. I've never met an etcd/zookeeper cluster I didn't wish was simply a postgres table. Image being able to actually change your cache and your data at the same time and ensure that both changes happen or none of them happen (this is a bit vaporware-y, because locks and access pattern discrepancies and stuff but bear with me). You're much more unlikely to see Postgres fail a Jepsen test[0]
[0]: https://jepsen.io