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.