system
Given the following post, respond as if you're commenting on Reddit:
human
[ASOIAF] Why didn't the Kingdom of Westeros break apart after the usurping of King Aerys II, but it did after the killing of Ned Stark?

The Targaryens supposedly represent a dynasty that has lasted centuries without interruption. Robert Baratheon rebels, and King Aerys II is eventually killed and the Targaryens dethroned. After which Robert is crowned king.

That seems enormously convenient for Robert. If a centuries old dynasty is suddenly usurped, would we not expect anyone with an even modest claim to the throne to come out the woodwork, and the lords of the seven kingdoms to make their move for independence? We know they do so later when Robert dies, but that seems like the point where it makes less sense.
gpt
Westeros hadn't really healed from Robert's Rebellion, it had only begun the process. By killing Ned, Joffery showed the country just what kind of King he would be. Robert was also not a great king, by all accounts he was a drunkard, spent coin like mad, and was barely active in the court. People were willing to give Robert a chance because he was still better than what they'd had before his Rebellion, but when Joffery showed his true colors, it forced bold men to take action. The instability was too obvious for players like Stannis, Renly, and Greyjoy to ignore, and Rob Stark had to avenge his father.