Apart from the query support, which others have mentioned, there's the sheer scale of data. The most recent time series implementation I worked on had to ingest

Apart from the query support, which others have mentioned, there's the sheer scale of data. The most recent time series implementation I worked on had to ingest terabytes per minute from a wide variety of sources in a single factory... And the goal was a single system combining every factory worldwide. Easily measured in exabytes per hour, with thousands of sources. A traditional relational database - even a really good one like postgres - just isn't built with that kind of use case in mind. There really is something to a database specifically designed for a particular kind of query on ridiculous quantities of data.