The real reason is that they have limited supply, and people want them. (people may not want bitcoin in the future btw) People always want gold and they want to gather as much as possible in their lifetime. It is the only solid asset which holds its true value through the centuries.
I do agreed that they have limited supply and I knew people want them. But I beg to disagree that in the near future people may not want bitcoin. Its a big no! instead, sooner or later people will realize how bitcoin is really a good thing to have it. and we know how popular it is now.