I'm not the architect of journald and wasn't really around when these decisions were made, so I can't really speak authoritatively on that particular topic.

There was mailing list discussion at the time journald was conceived though, you can find it if you look.

https://0pointer.de/blog/projects/the-journal.html might be a good entry-point.

SQLite here is okay, but DuckDB or LevelDB would be better. Either way, no need to invent a new storage format.