Deuteronomy 33:21
And he provided the first part for himself, because there, in a portion of the lawgiver, was he seated; and he came with the heads of the people, he executed the justice of the LORD, and his judgments with Israel. And he provided the first part for himself, because there, in a portion of the lawgiver, was he seated; and he came with the heads of the people, he executed the justice of the LORD, and his judgments with Israel.
Deuteronomy 33:21 stands within Moses' final blessing pronounced over Israel. That blessing contains no mention of Simeon. The words spoken about Gad in this verse give no reason for Simeon's omission.