My take, for what it is worth!

It is small potatoes! compared to wiping out all the Dons, Greene and Carlo in one sweep and the surprise element would have been lost otherwise
Too much of a risk There is always the possibility someone could let something slip....

I don't think anyone outside of Vito and Michael knew anything about the baptism murders, by design, even Tom was kept out of the loop
Nobody was let in on, what Vito and Michael were planning until their need to know after the traitor revealed himself

Then, after Tessio revealed himself as the traitor, whoever were picked for their specific assignments would have been assigned the task of tracking their respective targets' movements to plan out their hits

Once the plans organised and put together, the actual day of the massacre [the same day? as the Tessio brokered 'meeting' with Barzini] would have been picked and coordinated accordingly to carry out the murders pretty much simultaneously