Poinsettia was not popular in the United States or used during Christmas until the 19th century. They get their name from Joel Roberts Poinsett who brought the plant from Mexico to the United States in the 1820s. The original Aztec name of the flower is cuetlaxochitl.
The poinsettia quickly gained popularity because of their supposed relationship to the Star of Bethlehem. The petals of the poinsettia are said to resemble the star that led the Wise Men to baby Jesus. With the religious ties and long history—and the use of poinsettia sap to make latex!—it is not surprising that the poinsettia is such a popular holiday flower.