The North remembers, and Northerners don’t like the Targaryens.
The trailer also makes it clear that the North remembers what life was like under Targaryen rule.
It wasn’t so long ago that Ned’s father and brother rode south and were murdered by the Mad King, and the prevailing opinion is still that Rhaegar kidnapped and raped Lyanna Stark.
Jon has been crowned king, and Dany intends to seize the Seven Kingdoms, not Six Kingdoms that share a border with the North.
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Game of Thrones Season 7: Weeks Ahead Comic Con Preview (HBO)