"The formula that was struck down identified jurisdictions subject to preclearance as those with a history of a voting test or device and less than 50 percent voter registration or turnout as of 1964, 1968 or 1972."

http://www.brookings.edu/blogs/up-front/posts/2013/06/25-sup...

The decision hinged upon this very narrow point. Congress' decision to require preclearance was not declared unconstitutional on its own. The problem was that the formula for deciding who required preclearance is a static rule that does not allow for the evaluation of any events after 1972. Therefore, regardless of how much a state or other jurisdiction changes, they could not change their status under this law. If they were originally on the preclearance list, never had another voting anomaly, elected minorities to every position in the state, and had 100% minority turnout, for decades on end, they would remain on the preclearance list, because of what happened 40 years ago.

Chief Justice John Roberts even wrote in his opinion that congress is free to make new legislation that has the same consequences. But it must rely on current data to evaluate jurisdictions.