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?