Why is St. Peters name St. Peters?

Answer 1

He had two names which his parents gave to him. Simon (pronounced SeeMown) and Kefa, which was Petros when translated into Greek. He was a Jewish man. The word "Saint" is a translation of the Hebrew word Tzadik. Whether Simon Kefa was ever called a Tzadik within his lifetime might be an issue of some debate.

Answer 2

Peter refers to petra=rock.
The St. part comes from the catholic church.