Why did Saint Mark become a saint?
A "saint" in the bible is a person who believes in God through Jesus Christ. The apostle Paul mentions saints frequently in his writings. This is positionally.
Experientially, a saint is a person eminent for piety, and virtue: a consecrated person.
When Mark the writer of the Gospel according to Mark, first believed in Jesus Christ as the Savior he became a saint.
Later on the early church fathers gave him the title of Saint Mark.