Robin became an oiran, she had access to the Shougun Palace which is basically the QG. She was surrounded from Orochi, the Oniwabanshu and even Denjiro/Kyoshiro. A damn lot of info and even then the real info came from Wano citizens and the investigation of all the SHs combined (Franky as a carpenter, Nami acting as a ninja with Shinobu and so on). That manpower disposal and inner knowledge is completely different.
Killer worked as a hitman, he was basically given an order: "Kill X, Kill Y, Kill Z" he didn't know else and had no way to know more. Stop acting like, in the middle of a mission, he could go around and ask citizens for info (especially Infos in regards of a dangerous pirate who was recently imprisoned) and stop acting like they would give them to him of asked to. You're also forgetting that Kamazo was supposed to be a ghost, newspapers said that he used to act during the night for fast and clean jobs and his face was barely seen (the wanted poster was Just a silhouette).
Killer didn't have the means and the people to organize anything. You're climbing glass walls.