I believe it to be because of the Germanic people. Roman and Hellenistic societies were republican and democratic. When the Roman Empire fell, Germanic tribes swooped in and established kingdoms in Italy and Spain especially (Visigothic kingdom of Spain). For this reason, a lot of Spanish names come from Germanic origin (Gonzalo comes from Gundislav). Of course Germanic people didn’t invent the concept of Kings, but bringing it with them to collapsed parts of the Roman Empire helped.