You can store timeseries data in Postgres if you want to (and optionally adding extensions like Timescale). You can store it in key/value like Redis or Cassandra. You can store it in bigtable. You can store it in MongoDB. Obviously different scenarios require different solutions.

KDB is a relational database with row and columnstore with features for time series and advanced numerical analytics along with a programming language. KDB is a thing because of those abilities, whether you use it for time-series data or not.