Para Don Corleone, havia certas linhas vermelhas que você não cruza, e envenenar as pessoas do seu bairro com drogas era uma daquelas linhas vermelhas. Também não ajudava que Sonny falasse na reunião do jeito que ele fazia. Essencialmente, Don Corleone percebeu que não tinha mais influência entre os outros dons - eles sabiam não apenas qual era sua posição, mas também quem poderiam explorar para contornar isso.
Spoiler à frente para mais explicações:
Had Carlo not had Sonny killed in the tollbooth, it's entirely possible Sonny would have been convinced to go along with dealing drugs, even after the attempt on Don Corleone's life. Michael was the one wild card none of the other families knew what to make of him; as far as they were concerned, he was either weak like Fredo or just a simple civilian. That was what allowed them to think they could fool him into sitting down with a meeting with Barzini at the end of the film - they just didn't know that Don Corleone had warned him about that possibility before he died.