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)