[unixhero]

I would think that not having to pay the Oracle/MS/Sybase/Teradata licenses are a good enough incentive...

Speaking of economic incentives.

That could be a thesis topic for my hereto unwritten PhD in Behavioral Economics:

"Choosing to pay" What are the behavioral aspects of top leadership - Why do people still chose proprietary" A quantitative case study."

[Ygg2]

If I had to wager, it's support. Oracle/MS generally provide much better support that open source software.

[jeltz]

Oracle is definitely not known for their good support, and there are plenty of companies which provide really good PostgreSQL support.