Why is some tv programming focused on the end of the world?

Wwell...why is so much of the tv shows being aired, especially on National Geographic and the history channel focused on Armaggedon, the Apocalypse, Space as in relation to comets and asteroids destroying Earth, Alien life taking over, catastrophes,Prohets, Doomsday Calendars, Nostradamus as related to the end of the world and dire consequences...etc. I could go on...why can't they show more positive programming that is actually beneficial to our knowledge?