My personal theory is that the red matter had a temporal component. If all the red matter accomplished was to turn the existing supernova remnant into a black hole, it doesn't stop the shock wave from continuing on it's path of destruction. The shock wave is already gone. However, if red matter has a temporal property, then it would basically reverse all the effects of the original supernova. The shock wave would never have been created and the planet Romulas would be "un-destroyed." First the star supernovas', which threatens Romulas. Only then does Spock promise to save the planet, after the shockwave is already on it's way, "consuming everything in its path." After Romulas is destroyed, Spock shoots the red matter into the supernova remnant. I believe what happen next was one of those Star Trek "reset events." As a end result of Spock's actions, Romulas was fine. This next is from oldSpock's mind meld with nuKirk.