LW, money - all the money that was in drugs. They had to balance that windfall of money against the losses they would suffer as a result of a war. Don't forget that with Vito hospitalized and all that that portended about his future health and Sonny in charge of the family, the other families could feel confident that they would defeat the Corleones. They also might have figured that if they sided with Sollozzo, that the Corleones wold back down.