because it is a fake currency that is not backed by anything, nor is it tangible. They say it will be limited to 21 million bitcoins, but awhile back bitcoins were valued at .005 cents, so 20 bitcoins to make a penny. Now bitcoins are around $17-18 each? These type of sites are always improving and there will be a new fad or improved version of this type of site that people will flock too. The people that started this or got in early have made a shit ton of cash and at some point current/future members are going to get left holding the bag. Also i believe its expensive as shit to get yourself up to a decent mining rate, read people spend anywhere from $2500 to $10000 on their server farms/upgrades to try to earn more money + your electric bill will suck when your computer is running 24 hrs a day, 7 days a week at a high level.