Because more and more people are starting to wonder what the bank does to our money and they do not trust the banks anymore that is why they need virtual currencies like bitcoin. Because bitcoin is something they can control themselves and no bank can reach that.
And the other reason for the need if virtual currencies is that more and more companies are on a high speed are going to shift to digital form so it is the demand of that type of business that we must have some digital form of money.
From the sayings that " Necessity is the mother of Invention" I think its true for bitcoin or cryptocurrency also. Since we need a digital money which no one is controlling it, decentralized and part anonymous so this cryptocurrency wa invented or discovered. In addition the high cost of fees in sending transaction accross the globe.