To add to what Dirk wrote, it's important to remember that supporting multiple databases is very expensive.
Whenever a new feature is added in RDBMS A, someone has to port it and all future updates to RDBMS B, C and D. There's a huge complexity cost associated with this.
Most developers do their work with a single RDBMS. They might do the porting to others RDBMSes, but as far as I can tell they're not set up for cross platform testing. Folks working on the PG side might not notice this as much because that's where a lot of the new development seems to be. But it's very visible on the Oracle side. Pretty much every time I sit down to work on some new code, I have to debug the Oracle queries.
