The landscape was different. Digg was in 2004. Reddit in 2005. They both came in a time where social media was at it's infancy and it was anyone's game to make it big. Whereas today, there are already established social media sites and the best any alternative social media outlet can do anymore, is absorb some numbers and try to prove to be the better alternative. It's a lot about thinking outside the box and figuring what a platform can do that the other can't.