Well InfluxDB actually is a time series database. That's all it does.

The query language is optimised for time based queries. The storage is optimised for time series. It is part of the metrics/monitoring ecosystem so it has integrations with tools like Grafana.

PostgreSQL doesn't have any time series capabilities OOTB. It's just people modelling the concept in a relational fashion. No different to how Excel can be used as a time series database.

And seriously everyone should just get over FoundationDB. There are plenty of people who have invented new databases that got it right e.g. Cassandra, HBase, Redis etc.