The primary reason for me is that Postgres has stronger default constraints. If you care about keeping your data logically consistent then Postgres has more of that out of the box.

SQLite just makes the tradeoff to be simpler since often it doesn't matter. But don't make the mistake that it doesn't matter. Since PG helps avoid data problems and you might need to scale out web servers that is why Django for instance recommends switching to Postgres (or whatever you're actually going to use) ASAP cause there are differences. You may end up relying on PG to reject things SQLite doesn't care about by default. SQLite might let you get away with inserting data which PG refuses to handle.

Not to mention the DB specific features can differ. Like PG's JSON field types or etc.