Barzini well he -- he played this one beautifully.

I reckon, they deliberately concealed the Tattaglia connection so when Tom unearthed it, he would stop digging further which is exactly what happened

Vito Corleone: And what is the interest for the Tattaglia Family?
Sollozzo: (to Tom) My compliments -- You got me there but smug inside that it was playing out exactly as they had planned

I am a little worried about this Sollozzo fella. I want you to find out what he's got under his fingernails, ya'know but I too reckon, it wasn't until the meeting that his eyes were finally opened. “Tattaglia’s a pimp—he could never have outfought Santino. But I didn’t know until this day that it was Barzini all along.”

He was complacent, careless, underestimating Barzini
1. believing his no is final and they would just give up the lucrative drug business
2. sending Luca on that ridiculous mission to Tattaglia -- idiotic
3. and keeping his underboss and consigliere totally in the dark about it
4. not telling them about his no decision before the meeting
5. didn't boost up his security
6. didn't suspect why Paulie kept falling sick
7. And Fredo -- well -- Fredo was -- well -- his driver / bodyguard

Luca is the valued fear factor even into the Corleone future.
They need him, to help them get in and out of the car, carry their briefcase!

Tom: even Sonny won't be able to call off Luca Brasi.
Tom: Now what about Luca? Sollozzo didn't seem to be worried about Luca...
Sonny: if Luca sold out we're in a lot of trouble, believe me. A lot of trouble.