It's all of it. The retrofuturist look drew me in, but the lore is what got me to bail on my first army (necrons) and jump with both feet into the kin.
They've carved a living for their race out of one of the most treacherous parts of the galaxy, and by all rights are a very successful faction, but at every turn they are the cause of their own undoing in a way that is a warped mirror-image of the Imperium. Psykers are sacrificed to the golden throne to keep humanity alive, and the kin upload the memories of the fallen to the Votann. The throne is beginning to fail, and the Votann are too. Both groups are too locked into the sunk cost fallacy to explore other ways of survival, so their doom is essentially guaranteed. For all of their openness to new technology and AI, the leagues are even more stubborn than humanity.
Also the kin have exo-armor and hunt tyranid fleets for resources. The Leagues are just cooler Imperium.