Socially:
Some people don't trust governments to handle one of the most powerful collaboration technologies ever invented (money). All financial systems before Bitcoin were government controlled. Some have behaved in a trustworthy way, many have not. And over the longer term they all tend to mess it up eventually.
So these people set out to build an alternative that they believed governments couldn't control.
Technically:
The interesting key advance that made Bitcoin interesting and successful was coming up with an algorithm that solved the problem of getting parties that don't trust each other at all, to collaborate on maintaining a global ledger to everyone's benefits, without them having to even know about each other.
This is already a feature of money (I don't need to know about you to have indirect financial ties to you) but was not true of the financial system itself until Bitcoin.