Imagine that you found out tomorrow that your bank was going to start charging a storage fee on your cash. What would you do? I think most people would withdraw their money from those accounts and keep it in cash. It's unlikely they would spend or invest it, even though that's the purpose of imposing negative interest rates.
In an unprecedented move, the Fed undertook an extraordinary experiment in monetary policy. Unable to further target an interest rate, already at zero, the Fed began announcing a quantity of cash that it would inject into the financial system by purchasing large numbers of government bonds and other highly-rated securities, and replacing them with cash.