Why was Joseph not included in the twelve tribes of Israel?

On the contrary, he got two of them, even though his descendants had by far what were not the most numerous tribes (Numbers ch.2 and ch.26). He was given this honor because of his leadership, his forgiving of his brothers, his supporting the family in the famine, and his superlative righteousness.

Answer:

Joseph was considered to be so fertile that the descendants of each of his two sons, Ephraim and Manasseh were so numerous as to be their own tribes.