There are so many things to be considered in this discussion. Sollozzo wanted money, Tattaglia wanted respect, and Barzini wanted power. Sonny wanted his own operation. Vito wanted to preserve what he had. Tom wanted to serve Vito. For me, it still comes back to what Sollozzo wanted from Vito: his political and legal connections. So, why would Sollozzo want to murder Vito? Sonny being hot for his deal on drugs? The novel makes it clear that it was Vito's personal relationship with politicians and judges that made possible the favors and protection he received from them. Without Vito, those relationships vanish.

So, we have to look elsewhere for an explanation. TB and others opine that the attempted murder was incipient with Barzini and/or Tattaglia and their motivations. It appears that it has to be that. Sollozzo's murdering the golden goose makes no sense; it all comes back to Barzini.