I understand that Arthur as a figure originates from Welsh folklore, but later on you see it taken up by the English. Why did they incorporate a Welsh figure yet pay less attention to early Germanic leaders? Also what do the various stages of the mythos tell us about the people and times which contributed to this?
Geoffrey of Monmouth, who was Welsh, wrote the Historia Regum Britanniae (History of the Kings of Britain) in the 12th century, less than 100 years after the Norman conquest. He wanted to show that the Normans were more glorious than the Anglo-Saxons, and that the Anglo-Saxons had displaced a great people (only to be displaced by s greater people). It was hard to do that with the Britons, who were massively criticized by Bede, so Geoffrey turned to the Celtic Arthurian mythology. Although plenty of people at the time realized that he'd mostly made up that part of his history, the book was very popular and Arthurian legend entered the English (at the time Anglo-Norman) conscious.