Why do people care about something tragic for a week and then any later they don't give a...?

Think of all the things that happened and are still happening the Slaves gathering Diamonds in Sierra Leone. The Rwandan Genocide. Uganda's Former Dictator Idi Amin(R.I.P). The Earthquakes in Haiti & Japan. The list goes on and on. Yet we do nothing. Donating maybe just so we can feel good about ourselves.