In his new article, The Decline of Black Power in the South, The NY Times Thomas Edsall nails it.

The long-term importance of Republican success controlling the redistricting process is that it provides the party with a tool to counter the growing strength of black, Hispanic and Asian-American voters. Republican control of Congressional district lines in 2012 allowed the party to maintain a 34-seat majority in the United States House of Representatives while winning one million fewer votes than the Democrats over all.

As the United States moves inexorably toward becoming a minority majority country, the Republican Party needs every available weapon to survive what it perceives as a siege. The Shelby County v. Holder decision issued by the five conservative members of the Supreme Court gives Republicans even wider latitude to use the manipulation of district lines through “bleaching,” “packing” and “cracking,” in order to maintain its control over state legislatures. This, in turn, grants Republicans control of the House of Representatives.