Why is King Arthur considered as a figure of literature, not of History ?
The Arthurian legends, written by Sir Thomas Malory, were borrowed in large part from French legends based around Lancelot as well as the English poem, "Morte Arthur," written Alfred Lord Tennyson. The stories have very little basis in history and are mostly based around legend and folklore.
Mallory, in his attempt to write the definitive English epic, created king Arthur (which was a common enough English name). Since its publication many historians have tried to prove a historical basis for Malory's tales but without success. The Arthurian legends, however, have become a major source of inspiration for artists ever since then.