In the cases where the other brothers weren't taken out, it was mostly due to the fact that the mob didn't feel threatened, and/or the brothers made it clear and convinced them that they wouldn't seek any retribution. Salvatore Scoppa was clearly a threat, he wanted to wipe them all out. Andrea Scoppa claims that he was on bad terms with his brother, but it was still his brother, so you know how that goes. Whatever the case, I'm still not buying the bullshit story of a lone disgruntled hitman or hitmen.