Because it is completely new thing. Someone came and said put your cash and we will give you a string of digits and letters that you will hold in virtual wallet and that will be your money. All historical crashes had taught us to be sceptic and I don't blame anyone for being sceptic. Think about it. What if some day someone crashes everything, then what? It is new, it is unknown and it has nothing to do with everything that we have known so far. BTC is only 10 years old.