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She didn't want to be "Queen of the ashes".

Now - They cry out for their true queen? They drink secret toasts to my health? People used to tell my brother that sort of thing, and he was stupid enough to believe them. If Viserys had three dragons and an army at his back, he'd have invaded King's Landing already. Conquering Westeros would be easy for you. But you're not here to be queen of the ashes. No. We can take the Seven Kingdoms without turning it into a slaughterhouse. If the great houses support your claim against Cersei, the game is won. With the Tyrell army and the Dornish on our side, we have powerful allies in the south. I never properly thanked you for that.

Game Of Thrones, Season 7, Episode 2 (Stormborn)

She wanted an actual kingdom to rule rather than a few smoldering piles of stone.