(This is actually Guido responding; I don't normally lurk here and have no time to register for an account.)
I think the liberal Python license has been very important. The early user community was very vocal about this. For example, I expect that Digital Creations (now Zope Corp) would not have built their business on Python if it had been GPL'ed. (I can't speak for why Zope Corp chose the ZPL but I note that there are now a lot of other businesses who develop for Zope.)
But there are also good points for GPL; read what the MySQL folks have to say about it.
Public Domain doesn't even enter the picture.