Full-scale RDBMSs, especially Postgres, have lots of goodies that either SQLlite doesn't have (or which it does have, but which aren't so richly featured). Once you've gotten hooked on a few of these, the distinction will feel a lot more clear.
Meanwhile the tipping points in favor of SQLite seem to be embedded systems, and its whole "service-less" architecture and plain ease of use. Which is why it still gets lots of love, for those contexts.
https://www.sqlite.org/whentouse.html
https://www.sqlite.org/quirks.html
Full-scale RDBMSs, especially Postgres, have lots of goodies that either SQLlite doesn't have (or which it does have, but which aren't so richly featured). Once you've gotten hooked on a few of these, the distinction will feel a lot more clear.
Meanwhile the tipping points in favor of SQLite seem to be embedded systems, and its whole "service-less" architecture and plain ease of use. Which is why it still gets lots of love, for those contexts.