Supply and demand would be the main reason. Take super cars for example, Ferrari, Lambo, Bugatti, Porsche, McLaren, Pagani...etc. Aside from their trademarks, only a few units are usually manufactured. Making these cars even more expensive! Same with Bitcoin, if those 21 million BTC's are near to be mined, price will surely shoot up with more and more trying to get these last Bitcoins.