ALL LEATHER MUST BE BOILED

I don’t feel I have an adequate answer to this question. The thinking usually goes that the continued presence of the Kingslayer irreparably soiled the institution’s reputation, but I’ve always found that to be a bit of a dodge, since we’ve since learned of several other infamous members of the Kingsguard who didn’t seem to damage it in that way, including one who literally caused a civil war. Then again, none of them appear to have slain the king they were sworn to guard only to keep their job guarding his replacement — maybe that really is a categorically more severe offense in the eyes of Westeros’s knights, I don’t know.

But I think the situation with Jaime, while undoubtedly a part of the problem, isn’t the whole problem. There’s also the fact that the kingdom just underwent a civil war, during which the existing regime was successfully toppled. So in addition to nobility and knights needing to repair the wounds of war in their own lands the way they always would following a conflict, there’s the additional upheaval of a new ruling family, and houses great and small being raised or cast down depending on their loyalties during the conflict — the greatest such upheaval the Seven Kingdoms had seen since Aegon’s Conquest. This likely kept a lot of eligible candidates very busy elsewhere.

Add to those two factors the sheer number of replacements required. I could be wrong, but I think this is the largest number of simultaneous inductions into the Kingsguard since its establishment. You’d need to find five gifted knights who were ready, willing, and able to forsake their current lives, claims, titles, and sex lives, and irrevocably bind their fate to that of Robert Baratheon, all while serving alongside the only two survivors of the previous kingsguard: a creep who killed the last king, and a guy who was probably trying to kill you at some point during the war that just ended. And not only would candidates have to hit all those marks, they’d also have to be politically sensible choices, helping bind the new Baratheon dynasty to important houses without alienating any others, while remaining palatable not just to Robert but to at least a majority of his entire inner circle: Tywin, Cersei, Pycelle, Varys, Stannis, Jon Arryn, maybe even Ned. Talk about threading the needle. And you have to do this five times over!

The fourth and final factor: Robert’s just not a very good king. Why should he be any better at picking out a kingsguard than he is at anything else he does as ruler of Westeros?

Add ‘em all up and you come to the sorry pass we find ourselves at when AGoT begins. I don’t doubt that better selections were possible even given these constraints, but the constraints do at least offer some explanation as to why things got so bad so fast.