Why St. Joan of Arc is called [of Arc]?

Nope. That's her father's name. Please see:
http://en.wikipedia.org/wiki/Name_of_Joan_of_Arc
Joan of Arc did not come from a place called Arc, but was born and raised in the village of Domrémy in what was then the northeastern frontier of France. In the English language her first name has been repeated as Joan since the fifteenth century because that was the only English equivalent for the feminine form of John during her lifetime. Her surviving signatures are all spelled Jehanne without surname. The surname of Arc is a modern transliteration of d'Arc, which itself is a nineteenth century French approximation of her father's name. Apostrophes were never used in fifteenth century French surnames, which sometimes leads to confusion between place names and other names that happen to begin with the letter D. Based on Latin records, which do reflect a difference, her father's name was more likely Darc. Spelling was also phonetic and original records produce his surname in at least nine different forms.[1]
To further complicate matters, surnames were not universal in the fifteenth century and surname inheritance did not necessarily follow modern patterns. Joan of Arc testified at her trial that the local custom in her native region was for girls to use their mother's surname. Joan of Arc's mother was known both as Isabelle Romée and Isabelle de Vouthon. No surviving record from Joan of Arc's lifetime shows that she used either her mother's or her father's surname, but she often referred to herself as la Pucelle, which roughly translates as the Maiden. Prior to the mid-nineteenth century when Jeanne d'Arc and Joan of Arc became standard, literature and artistic works that refer to her often describe her as la Pucelle or the Maid of Orléans. Her native village has been renamed Domrémy-la-Pucelle in reflection of that tradition.[2]