Here's why:
* Transportation was a common form of punishment in England from the 17th to the 19th centuries.
* Convict refers to a person found guilty of a crime.
Therefore, someone serving time in an English prison who was then sent to the colonies to work would be referred to as a "transported convict". They were often sent to places like Australia, America, and other British colonies.
Let me know if you have any other questions!