If Danaerys from Game of Thrones could destroy the entire city by herself with only one dragon, why didn't she attack and destroy King's Landing when she first landed in Westeros, had 3 dragons, and there were no giant crossbows?
She didn't want to be "Queen of the ashes".
(Daenerys) - 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.
(Tyrion) Conquering Westeros would be easy for you. But you're not here to be queen of the ashes.
(Tyrion) 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.
(Daenerys, looking at Varys) 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.
Because she has been poorly advised by her council. Or, at least that's the in-universe explanation. The real explanation is poor writing by D.B. Weiss and David. Benioff.
When she first arrived to Westeros, the first thing she wanted to do was take King's Landing, but for two reasons, it was postponed.
The first reason is that Tyrion told her that it would be better to leave King's Landing be for now and focus on other targets in Westeros (such as Casterly Rock), since it wouldn't be possible to take King's Landing and the Iron Throne without heavy civilian casualties. Tyrion was wrong. The latest episode demonstrated that the power of a dragon (let alone three) is more than enough to take down the armies defending King's Landing, take down the fleets surrounding King's Landing, and take down the Red Keep itself, without necessarily hurting any civilians. Remember that civilians only started dying in the latest episode in large numbers when Daenerys herself started targetting them.
So Tyrion was wrong and Daenerys could easily have taken King's Landing with minimal casualties in no time, especially considering that when she first arrived at Westeros, her army was much, much stronger than it currently is (she had 2 more dragons, and the full Unsullied and Dothraki armies).
The second reason it was postponed was because of an apparent threat by the army of the Night King. However, this was a poor reason as well. Firstly, because the undead army was not an immediate threat at that point. They were still beyond the Wall, and while Daenerys help would definitely be needed in mounting a defense once the Wall was breached, that had not happened yet. When you then factor into the fact that Daenerys could take King's Landing in a matter of hours, establish herself Queen, and unite all the forces of Westeros, and only then join Jon in fighting the undead army, wouldn't that be much better? It would.
In short: she didn't take Westeros immediately because Tyrion is an idiot and because Jon convinced her to fight the undead army prematurely even though she had plenty of time to take King's Landing first.
Burning innocent people who had no choice about where they wound up living would have gone against her stated goal of liberating the common folk from mad despots and tyrants.
Daenerys Targaryen : Lannister, Targaryen, Baratheon, Stark, Tyrell they're all just spokes on a wheel. This ones on top, then that ones on top and on and on it spins crushing those on the ground.
Tyrion Lannister : It's a beautiful dream, stopping the wheel. You're not the first person who's ever dreamt it.
Daenerys Targaryen : I'm not going to stop the wheel, I'm going to break the wheel.
And there are several time in the show when she does want to go into burn-overkill mode (against the rebelling slaver cities and then King's Landing after her setbacks at sea and Highgarden), and she gets talked out of it by her advisors, so it's not like the thought never occurred to her.
Well she could have, but:
- She didn't fully know what she was up against at that point. How could she be sure they didn't have any anti-dragon defense.
- She decided to help Jon first, if she didn't she could have gotten there sooner
- At that point she listened to here advisors and had some form of.. morale
- Wouldn't have been a fun couple of seasons if she did, would it?