The other reason the Don might have been slightly slower to see Barzini's involvement was the Don's prejudice against such "dirty" businesses as drugs and prostitution.
Since Sollozzo was a former pimp and Tattaglia was deeply involved in pandering, the Don may have simply conflated both vices together and not seen past Tattaglia's involvement, for whom he obviously had no respect.
The Don might have had these thoughts:
"This narcotics business is just the sort of infamia a man like Tattaglia would be involved in. Barzini's no friend of mine but he's no fool either. He's too smart to waste his time backing a drug peddler like Sollozzo. But Tattaglia is greedy and stupid."
In the novel, Vito says almost exactly that, after Tom briefs him on the upcoming meeting with Sol. He says something to the effect:
"Do you have it in your notes that Sollozzo made his living before the war from prostitution--as the Tattaglias do now. Write that down before you forget it." Tom is stung: he knows that Vito is "notoriously straight-laced on matters of sex," and fears that he will let that bias get in the way of ok'ing a deal that Tom has just touted as being good for the family.
