Tamales are a Mexican Tradition however Tamales actually originated from Mesoamerica and not Mexico itself.
The Mesoamerica is a historical region and cultural area in North America. which extends from approximately central Mexico through Belize, Guatemala, El Salvador, Honduras, Nicaragua, and northern Costa Rica.
Tamales have been eaten by Mexicans for 100's of years since the 1800's.
Mexicans also eat Tamales as a tradition on Christmas as well just like Americans usually eat Turkey or Ham on Christmas the Mexicans prefer to eat the Tamales.
The Tamales symbolize the birth of the corn god.
Since Tamales use corn husks they are a good food for symbolizing the birth of the corn god so they became a Mexican tradition.