Why did the President decide to nominate Roberts for Chief? The first reason is obvious--the way things are shaping up, he seems like an easy confirmation.
But allow me to propose a second, more speculative possibility. A distinguishing characteristic of this President seems to be the faith he puts in his own personal judgements and assessment of people. Perhaps it is arrogance, perhaps it is that he truly is a better judge of character and ability than the rest of us, but he truly seems to believe that he has better judgment about others than anyone else around him. Or perhaps he wants someone who he thinks will be loyal to him and no one else (such as outside interests). It is similarly my impression that far more than most Presidents he relies on his personal assessments of people who he chooses for his inner circle, rather than their resumes or experience. Indeed, he chose Roberts notwithstanding his relatively short time on the bench. Roberts, of course, was selected by him as well. Let me suggest that Roberts therefore has the one necessary (but not sufficient condition) for being Chief--he had previously won the President's trust the first time around.
Recall, for instance, when he claimed to have seen into Putin's soul so as to be able to judge the man? It is also my impression (based in part on my time in the Administration) that when vacancies occur, this White House has tended to promote internally more than most administrations, even to the extent of promoting relatively junior people to relatively senior positions. When Roberts was appointed, it truly was a utter surprise, even to those very close to the White House who were reported to be closely involved in the selection process.
If I am correct in this assessment of the President's decision-making style, this would suggest that his next nomination would likely be from the crop of judges that he has appointed since becoming President. This would include Brown, Clement, or McConnell, but not more experienced luminaries such as Luttig, Jones, or Wilkinson.