Horizontal scalability. Outgrowing a single master database (writes being bottlenecked, not reads) is really painful. You can engineer around it, but spinning up another master in a multimaster DB is a really quick and easy fix (once you have tooling built out it's a matter of minutes). Sharding your Postgres DB is not a quick and easy fix. It will likely take about a month to engineer around and perform the data migration, if not longer.

I've heard good things about Citus for horizontally scaling Postgres.

I've pretty much only heard about it in the context of OLAP. If your data model requires transactions, you're gonna have a hard time when you outgrow a database.