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.