Por que os eventos de The Purple Wedding não ocorreram antes?

Eu não acho que isso seja explicitamente declarado nos livros ou programas de TV, mas minha interpretação foi esta:

Lady Olenna sabia de suas conversas com Sansa que

Joffrey was a monster, and that she did not want Lady Margery married to such a man.

No entanto,

she still wanted Margery to become the Queen. The only way to achieve both goals was to kill Joffrey. However, if Joffrey died after the wedding, the Tyrells would be the first ones Cersei accused: she'd recognize that they got everything they wanted, and discarded her son once they were done with him.

Então, a melhor solução foi

kill Joffrey at the worst possible time for the Tyrells: right before the wedding. This way, Joffrey is eliminated without any fingers pointed their way, and Margery is in a position to be betrothed to the mild-mannered Tommen.

Spoilers para o livro (eu não sei como a série de TV trata isso, então não posso resolver isso):

Olenna didn't plan the assassination by herself; it was a conspiracy of her and Littlefinger and possibly Margaery Tyrell. I don't believe that it was Olenna who chose the occasion, but Littlefinger who chose it. And he chose it because the occasion of the wedding, with its mass drunkenness and mass confusion, made it easier to blame Tyrion and spirit away Sansa. Why did Olenna go along with it? We don't know what Littlefinger said to her, but we do know he can be very persuasive.

Isso é especulativo, mas pela natureza de um casamento, muitas pessoas podem se aproximar de Joffrey. Isso gera suspeita em um grande número de pessoas e permite que os conspiradores tenham uma negação plausível (na pior das hipóteses) ou desviem a culpa para outra parte (como Tyrion).

Pode ter sido difícil arranjar outra oportunidade que lhes permitisse apontar a culpa para eles mesmos. Afinal, o assassinato teria feito pouco bem ao Mindinho se

Sansa was imprisoned for the assassination and beyond his reach

ou para os Tyrells se

Lady Olenna was accused. This might even cause the Lannisters to have the wedding annulled and/or remove Tyrells from the small council.