First, the Don was a lot older than Hagen, and by being around longer, he had a larger network of contacts.
Second, in the grand Sicilian tradition, he was a "personal" Don--that is, he was available and accessible to people, who probably called him or talked with him at every opportunity. Remember that he upbraids Bonasera thus: "I can't remember the last time you invited me to your house for a cup of coffee." Not idle talk: he probably would have gone to Bonasera's if Bonasera had invited him. So I have to assume that the Don had a constant stream of visitors who kept him up to speed.
Third, probably a lot of the old-timers among the Don's constituency didn't feel comfortable around Hagen, for obvious reasons.
Fourth, the Don probably encouraged all of these contacts as a way of constantly checking Hagen's loyalty as well as his judgment.