I don't know exactly why. Maybe because we have access to so much media and information due to changes in technology (iphones, tablets, wifi) that people develop new belief systems easier. Older rules about tradition aren't as strong as they once were and people value personal freedom and change more than they value keeping traditional beliefs going.
My favorite new religion is Jedism. It's a newer religion based on the teachings of Jedi from the movie Star Wars. It's very fascinating even though I wouldn't want to join the movement. With more technology, information, and media at our fingertips people are able to form religions from all sorts of sources.