I suppose this is partially philosophy and partially theology, but perhaps its more likely Historians can give a factual answer:
As main part of the Ontological argument for the existence of God, it's posited that God must existed due to existence being a feature of perfection. The rebuttals to the Ontological argument typically attack the idea of existence itself being a feature, or the layout of the premises. However, what I notice is that the idea of perfection being necessarily a feature of God is never a focus of questions. This seems to me to be very odd. How did we get from the Old Testament God, who is wrathful and jealous according to the Bible itself, all the way to God being omnipotent, omniscient, omnipresent, and omnibenevolent? It doesn't seem at all obvious that any of these traits need to be inherent to any entity with power beyond our understanding.
If no one else provides a more complete answer I'll come back after my class and elaborate more but something to keep in mind here is the great influence of Neo-Platonist philosophy on Christianity. This influence took the form both of Christians being directly influenced by pagan Neo-Platonist philosophers and also a slightly more indirect approach wherein Christians would be influenced by Hellenized Judaism (such as that taught by the great philosopher Philo of Alexandria) that was itself heavily influenced by Neo-Platonism. Additionally, Aristotelianism also formed some influence especially if youre curious about the context Anselm lived in.