Why did Bitcoin first start to gain economic value?
Satoshi created a system that:


Allowed users to trust transactions without having to trust any single entity.
Opened it up so that anyone could participate and exchange computation power for Bitcoins.
Is designing with a fixed size (21 million Bitcoins), he created an incentive for users to get involved early while Bitcoins are relatively cheap to generate; there is less risk of future inflation reducing the value of early adopters.