Possibly Sauron's influence, which might have been greater than even Saruman himself knew. By the time Pippin picked up the palantir, we know it had been some time since Saruman had made a report to Sauron. I suspect this is because after he told Sauron he had Gandalf imprisoned, his own mind was in turmoil, his desire to have the Ring for himself warring with his "loyalty" (more likely fear) of Sauron. Moreover, it was to Sauron's benefit to encourage Saruman to stay put; in Orthanc, he could use the palantir to keep tabs on him and worm his way into his thoughts, exerting control from afar, also providing Sauron with an allied stronghold behind enemy lines, so to speak. Keeping Saruman there and under his influence gave Sauron an advantage he would not have had, if Saruman had never looked into the palantir and been drawn into Sauron's web. He would have been a rival, but one free from any ties to Mordor, which ultimately could have made the situation for the West even worse.
That aside, I think there was a hefty dose of fear involved as well. At the same time that Saruman spoke with the Nazgul at Isengard, it is said that from that meeting, he perceived the full horror of service to Mordor, which nearly made him switch his allegiance back to Gandalf and the West. Once he lost his valuable prisoner (and became angry because of it), he was definitely in a pinch. If he remains in Orthanc and reports in to Sauron, he will certainly face the Dark Lord's wrath for allowing Gandalf to escape (which he likely could not have concealed from him, which would explain why he had not reported for so long). Staying there and keeping silent, he at least retained some semblance of power. But if he leaves -- particularly after he knows the Nine are abroad and searching for the same thing he wants -- he is in danger from both sides, the old and new allies he has betrayed. If he had known exactly where the Ring was and who had it, he might have taken the risk of riding after it, hoping to get it first so that by the time the Nazgul caught up with him, he would be the new Ring Lord -- but that was pretty chancy, since he didn't know more than "in the Shire."
Did that make any sense? Maybe not. Head's still ringing from last night's concert. And a Merry Christmas Eve!
