because there are only 21 million coins (about 16 mil mined so far) and there is 7.5 billion people in the world.
with simple math with a very small portion of population adopting bitcoin you can figure out how much bitcoin will be worth in a couple of years (like 5 to 10 years).

also looking back you can see bitcoin started from being traded at 10 cents and less and slowly and sometimes rapidly grew to be worth >$1100 today and it has not even slowed down overall!

although i should add that just holding is not a good idea. you should spend something and see the benefits of spending bitcoin and i promise you that you will love it. also try services such as Gyft and Purse to get huge discounts and save up on lots of money.