Why did the U.S. Supreme Court dismiss Hollingsworth v. Perry without deciding the constitutionality of Proposition 8?

The U.S. Supreme Court dismissed Hollingsworth v. Perry in 2013 because it determined that the proponents of Proposition 8 lacked legal standing to sue to appeal the lower court's ruling.1

Under Article III of the U.S. Constitution, a party must demonstrate a "personal and tangible harm" that a favorable judicial decision would be likely to redress to maintain an appeal. The Court held (5–4) that the private citizens who defended the initiative had not suffered such harm, as they were not the state officials charged with enforcing the law. Because the proponents lacked the necessary standing, the Supreme Court vacated the Ninth Circuit’s decision and remanded the case for dismissal, thereby leaving the district court’s ruling—which declared Proposition 8 unconstitutional—in effect without the Supreme Court itself issuing a decision on the constitutionality of the measure.1