If Luca had been alive to hear that Don Vito had been shot, Luca would have gone on a rampage that nobody, not even, as Tom said to Sollozzo, Sonny would have been able to control. Luca may not have been able to get to Sollozzo, but given that he probably knew that the Tattglias were involved, he would have run roughshod over that entire family. Eventually, someone would have been able to take Luca down, but it would have been a bloody road to that point.
Conversely, if Luca had been hit too early, that would have tipped off Don Vito that Sollozzo was not finished with the Corleone Family and he would have been too atuned to a potential threat for Sollozzo to be able to get to him.
That's why it was crucial to coordinate the hit on Luca with the hit on the Don.

Sammy The Fish