My experience has shown me that Companies will buy/use technologies based on two pillars, Trust and Value. The pillar of trust takes an enormous amount of time to establish and can be decimated very quickly. Trust is about 95% of any buying relationship. Companies will even buy products they don't understand the value of because they trust the vendor it comes from.
PostgreSQL doesn't have the network of value added resellers and internal sales reps that Oracle has amassed. These are people that engineers dismiss but management often see them as a support tier for their customers of a product. That tier builds trust.
If you try to introduce your product/service to an enterprise B2B market with an established market leader and intend to sell or distribute entirely off of value alone you will lose nearly every time. In smaller companies that can't dream of licensing Oracle the risk is worth the reward, it's not like they have a choice and they then grow an in house knowledge to be that support tier if they scale.
Consider that I f something breaks at a big enterprise it's likely to cost millions of dollars in lost payroll while it gets fixed. Big companies would rather mitigate the risk and buy a supported and proven product. They trust it.