I think they got tired of having folks argue with them about the license.

In short though - was developed at berkeley so released under a BSD license. The license has served them well (there are a lot of commercial entities supporting postgresql development, and lots of others with various forks). And finally, it would be hard to change at this point and they don't see a gain.

A note - in contrast to Elastic, there is no one copyright holder with postgresql. So this makes it much hard to get a relicense to for example a situation where one commercial entity has a special right to the terms / relicensing power.

That doesn't sound quite right: MIT/BSD licenses are GPL compatible; you could trivially release MIT code under the GPL. I believe that's what https://www.hyperbola.info/ is doing, for example.

It's compatible, but if Postgresql wanted to go harder copyleft and switch the license for future releases, they'd need to get pretty broad agreement, and that would be difficult, there are a lot of corps / enterprise players in the ecosystem, and things like (a)GPLv3 are often totally banned. Is GPLv3 software even allowed in the Apple iphone App Store? The Windows Phone app store?

GPL is a big hammer you can use to force back contribution into the project and ensure that folks that use the software actually contribution to it continuing to be available and relevant - if Postgres has been able to sustain themselves for all these years with a more permissive license then why would they risk going GPL and scaring off a lot of their customers just to ensure that each customer that remains can be forced to back-contribute any innovations they make.

(please note that GPL doesn't in actuality force anyone to back-contribute but in effect it tends to work that way - it actually puts a lot of responsibility on the original authors to re-acquire improved versions of the library but it does ensure that those improved versions are accessible in some manner)

GPL does not necessarily ensure that those improved versions are accessible to the original project, people receiving the source code to improved versions could simply keep those versions secret. See the controversy around grsecurity for an example of this, although one old version of their code did get leaked.