I think they nominated Roberts because the Senate, bullied by Chuck Schumer, was making idealogy an issue in Judicial confirmations. Schumer was running around yelling "extreme" to any of Bush's nominees, including Janice Roders Brown, Miguel Estrada, and Charles Pickering.
Roberts appeared smooth, not extreme, and his hearings where he did not refer to any notes impressed many people.
He was selected to cancel the "extreme" charge from Schumer.
-PJ
Bottom line is he wanted Roberts on the court from the get-go.
Just a little history.