#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.

<sub> Game Of Thrones, Season 7, Episode 2 (Stormborn)</sub>

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