Compatibility with established Oracle apps, and a whole lot of database-peripheral stuff (much of which has better alternatives, but those are often newer, and we're back to legacy compatibility.)

Some of this (more on the DB than peripheral part) you can mitigate by buying Postgres Plus, which is a proprietary enterprise version with, among other things, extensions for Oracle compatibility, and it's still vastly cheaper than Oracle. But no one (well, except maybe MS, who can leverage Windows and .NET to sell SQL Server) is close to Oracle when it comes to selling RDBMS software to enterprises.