Still, Sonny doesn't decide to hit Bruno until the hospital thing which occurs just hours before Bruno is hit. So, they must have gotten lucky with that hit. Composing a list of hits is not the same thing as carrying them out especially when it is a hit on one of another family's top members.
And planning hits right after Vito is hit: they are still composing the list at that point.
As I posted above, according to the dialoge in the first scene, they are composing a hit list and there is some objection, openly by Tom, as to the list being to big, too personal, and initially Bruno was probably removed from that list.
But the line " After the hospital thing...." indicates to me that although Tom may have originally convinced Sonny to remove a few names from that list and just go after the Turk, after the "hospital thing" Sonny did indeed get hot and probably said "Fuck this consigliere, now that they tried again to kill my father and my brother was hit by a crooked cop, all bets are off! Everyone is fair game. I want someone's head for this!"
And the word went out, and they probably got lucky, and took out Bruno.
