If Rueben and Ephraim were the firstborns, why didn’t they get the king?

Reuben defiled his father’s bed, so he was excluded.

Ephraim was called the firstborn, but their representatives failed to rebuke their younger brothers Dan, which resulted in the slaughter of a village, and Benjamin which resulted in the violent death of a woman, and near genocide of the tribe of Benjamin. God later demonstrates through Samuel that a man of God is perfectly capable of giving a rebuke in the conditions Ephraim was given. Even so, prior to Saul, Samuel was judging Israel, being born from the tribe of Ephraim.

Simeon and Levi were both excluded for slaughtering a village after their sister Dinah was violated. And this leaves Benjamin, which is where the first king came from, Saul; and then Judah, David’s tribe. 

References

Genesis 49:4

Jeremiah 31:9

Judges 18:2

Judges 19:1, 16, 22-24

Judges 18:17, 25-27

1 Samuel 13:13-15

1 Samuel 1:1-2

Genesis 49:5-6

Genesis 34:25

1 Samuel 9:21

1 Samuel 17:12