Quoted in this LA Times article:

So are the 8-1 decisions signs of a new harmony at the Supreme Court, or examples of smart, tactical moves by the chief justice?
Stanford University law professor Pamela Karlan says it is the latter. “He didn’t have the votes” to overturn the Voting Rights Act, she said, so Roberts opted for a decision that weakens the law.
It takes five votes to have a majority at the Supreme Court, and many lawyers questioned whether Justice Anthony M. Kennedy would supply a fifth vote to strike down the key part of the Voting Rights Act.
“Kennedy is sensitive to the racial dynamics. And they all knew the court would take a huge hi””had they struck down the law, Karlan said. “That decision would be very hard to explain to the American public. The message would have been: ‘Conservative activists strike down voting rights,'” she said. “Besides, Roberts is young, and he’s in it for the long haul. He can afford to wait.”