What happened to Dan & Ephraim in Revelation 7?

Q. Why are Dan and Ephraim not included in the list of Jewish tribes in Revelation 7?

What a great question…since it reveals that you are reading your Bible closely. Yes, the list of the tribes of Israel that you read in Revelation 7 is different from what you would normally read in the Old Testament. It is a section with several interpretive possibilities. I would suggest the following solution.

John’s theme in the Book of Revelation is to pastorally encourage Christians (those who by personal faith in Jesus Christ’s redemptive sacrifice on the cross and subsequent resurrection) to overcome the idolatry and sexual immorality of the world and remain faithful to the Lord Jesus Christ. The tribes of Dan and Ephraim were known for their blatant idolatry in the Old Testament. Their exclusion from this list of tribes, each with 12,000 people sealed, is likely based on their close ties with idolatry. Alan Johnson insightfully writes,

“The early church held that the Antichrist would arise from the tribe of Dan. Charles has argued that this belief is in fact pre-Christian Jewish tradition, first mentioned in Christian sources in Irenaeus (d. second century A.D.) … Furthermore, Dan was associated in the OT with idolatry (Judges 18:18-19; 1 Kings 12:29-30). This may be the clue. If John sought to expose Christian idolatry and beast worship in his day by excluding Dan from the list of those sealed, it may also be possible to explain, on the same basis, why Manasseh and Joseph were chosen to fill up the sacred number rather than Manasseh and Ephraim. In the OT Ephraim was also explicitly identified with idolatry (Hosea 4:17).” (Expositor’s Bible Commentary, Volume 12)

To go beyond your original question, about why the omission, it may be informative to consider that many scholars identify the 144,000 with the whole elect people of God, including both Jews and Gentiles. If this is accurate, then this passage, using symbolic numbers, refers to the sealing of the true church from out of the midst of those who merely claim to be the church. Let me conclude with another quote from Johnson,

“The description of the judgments under the sixth seal (6:12 ff.) ends with the question "The great day of their wrath has come, and who can stand?" (6:17). Chapter 7 answers this question by implying that only the true servants of God, who are divinely sealed, can be protected from the wrath of God and the Lamb.” (Expositor’s Bible Commentary, Volume 12)