Common custom among royalty was to name firstborn son either after father or grandfather and second son after father (unless firstborn was named after father already). With younger sons there was more variation-name could be determined by day of birth (like with John Albert of Poland, born on 27 December, feast of St John the Evangelist) or parent(s)' favorite saint (like with Sigismund of Luxembourg). Lack of saints named Arthur could explain lack of Arties on the throne, although once introduced, say by surviving Arthur Tudor, long lines of Arthurs are likely due to his son/grandson being named after him.