Memes

I think it's your third point, mostly. Maybe even as sharply stated as: boomers became more conservative as the system and status quo brought them wealth, comfort, and security, so naturally they wanted to keep that going. For the generations that have followed, the system and status quo have only continued to bring those benefits to the boomers, not to them, so they're less likely to trend conservative to perpetuate a system that has failed them.

Additionally, in the years since the boomers came of age, the political right has moved away from a traditional conservative platform to a very extreme and hateful version of itself. Even if many millennials had shifted slightly to the right as they aged, the party typically associated with conservatism has moved so much farther to the right that even with their gradual shift, these millennials are still far closer to the left, or at least to "not whatever the right is saying".