You have to keep in mind that even the Kings of Judah sacrificed their children to the fires of Baal, as did members of other tribes. The reason why Dan was banished is much deeper than simple idolatry. For all of the tribes were guilty of that.
Both Dan and Ephraim are not mentioned in the list of the 144,000. Both Dan and Ephraim had idolatrous golden calf altars built by Jeroboam. The altar in Ephraim was at Bethel.
I'm going to try and avoid Christian identitarian sources and am looking for Jewish scholarly sources on my presumptions :
A. Dan is spread throughout the bloodlines of the "white" European peoples as well as some Persians. http://britam.org/dan.html
B. Dan was the first tribe to commit idolatry. That Idolatry was so severe that they stole idols from pagans. And they continued that idolatry until the exile. https://www.cgg.org/index.cfm/fuseaction/Library.sr/CT/BQA/k/115/Wh...
D. I personally believe that Dan was exiled not only because of their idolatry while in Israel, but that they refused to keep any part of YHVH's commandments or ordinances while in exile. They spread throughout Europe, interbred with the local peoples. They became a war mongering and conquering people. Continuing not only in idolatry, but murder, rape, ect. Think of the Vikings or Scotts and their ancient ways. Think of the sacrifices to Odin, or the blood eagle. Dan became monsters.