Because Hellenized Judea was bilingual and influenced by Greek culture. Even as early as the Maccabean period, we find many examples of Jews replacing their Hebrew names with similar-sounding Greek or Latin names (Simeon → Simon, Menahem → Menelaus, Eliakim → Alcimus, Joshua → Jason, Reuben → Rufus, Joseph → Justus, Silas → Silvanus, etc.), or bearing double names that pair a Hebrew name with a Greek one (e.g. Judah Aristobolus, John Hyrcanus, etc.), or having Greek names that translate the meaning of the Hebrew. So in the NT Peter's Hebrew name Simeon was Hellenized as "Simon", and his Aramaic nickname Cephas had a Greek counterpart "Peter" which was often coupled with his Hebrew name as a double name (Symeon Peter, or Simon Peter). Similarly, Tabitha is an Aramaic name and Dorcas is its Greek translation, and Didymus is the Greek equivalent of Thomas. Similarly, "Christ" as a title was the translation of the Hebrew title Messiah. The nicknames however were often not part of a double name but set apart as an alias: "Saul who was also known as Paul" (Acts 13:9), "Simon who is called Peter" (Matthew 10:2), "Thomas who is also called Didymus" (Acts of Thomas 1), "Jesus who is also called Justus" (Colossians 4:11), "John who is called Mark" (Acts 12:12), and of course "Jesus who is called Christ" (Matthew 1:16).
For a detailed discussion, see Richard Bauckham's Jewish World Around the New Testament, 2010, pp. 371-391.