[Inziladun]

Originally Posted by Faramir Jones

The answer is very clear: Saruman was a rebel. When he was given the tenancy of Orthanc, still Gondorian territory, by Ruling Steward Beren, Denethor's ancestor and predecessor, it was on the understanding that he would accept his obligations as a tenant. Saruman later rebelled, effectively declaring his independence of Gondor. Why should Denethor, or any of his predecessors, trust Saruman, let alone let him into any of their confidences? This is even before Gandalf found out, due to his imprisonment, that Saruman was an enemy as well, in league with Sauron.

Well, it's indicated that Saruman had been well received in Gondor even up to Denethor's time. Denethor himself told Gandalf that Saruman had been searching the archives in Minas Tirith (for records of the Ring).