"Worst performing" is misleading here due to the novelty, speculation and media bubble perfect storm that occurred in late 2013. You can't compare it to national currencies.
Is it a currency you should store long term? No. Is it useful as an intermediate currency for digital transactions where privacy is needed? Absolutely
Then people who trusted Bitcoins sold them and traded for dollars or other fiat currency because....?
What if the grid goes down and stays down for a while? Like natural disasters, terrorism, widespread insurrection, nuclear war, zombie epidemic, alien invasion, pick your favorite catastrophe. What good are your bitcoins then? They’d be more useless than paper money, which would at least be valuable as toilet paper in any one of the aforementioned scenarios.