Moses died circa 1406 B.C. In order to understand why Simeon is not mentioned in Deuteronomy, we must go back in time to when Jacob, the father of the twelve sons who were the progenitors of the twelve tribes of Israel, died (circa 1859 B.C.). The record of Jacob’s blessings is in Genesis chapter 49. The relevant section is in verses 5-7:
Simeon and Levi are brothers – their swords are weapons of violence. Let me not enter their council, let me not join their assembly, for they have killed men in their anger and hamstrung oxen as they pleased. Cursed be their anger, so fierce, and their fury, so cruel! I will scatter them in Jacob and disperse them in Israel.
The New International Study Version notes give this explanation with regard to Jacob’s prophetic words that Simeon’s descendants would be scattered:
Fulfilled when Simeon’s descendants were absorbed into the territory of Judah (see Joshua 19:1, 9) and when Levi’s descendants were dispersed throughout the land, living in 48 towns and the surrounding pasture-lands.
At the time of the second census conducted by Moses and Eleazar, son of Aaron the priest, the tribe of Simeon was the smallest and weakest of all the tribes that came out of Egypt, only 22,200 men aged 20 years or more (Numbers 26:14).
Simeon was cursed by his father Jacob, probably because of the event recorded in Genesis 34:24–30 when Simeon and Levi took revenge on the men of Shechem after the rape of Dinah. After the horrific event, Jacob said to Simeon and Levi,
You have brought trouble on me by making me a stench to the Canaanites and Perizzites, the people living in this land. We are few in number, and if they join forces against me and attack me, I and my household will be destroyed (Genesis 34:30).
Moses was undoubtedly aware of this and that may be why the tribe of Simeon was excluded from Moses’ blessing.