The matching of Yaakov and Leah separated the component of the Mashiach Ben David, and their son Yehuda was born with the power of the Mashiach Ben David. Rachel took from Yaakov the component of the Mashiach Ben Yosef when she had Yosef. The reason why it's not called the Mashiach Ben Yehuda is because he sinned with the sale of Yosef. Yosef accomplishes his purpose through leadership, and leadership is a quality that can be attached to any man. The Mashiach Ben David performs his job through kingship, and kingship is dynastic.