Because QE is an active tool to support credit liquidity and they determined that markets were liquid enough to remove that support.
EDIT: And just to be very clear to the 2 people who read this comment, maintaining a balance sheet is still market support b/c you still buy treasuries on the open market to offset the principle of your existing treasuries that reach maturity. So stopping the growth of the balance sheet just means you're not accelerating support. Tapering is the thing that you do if you're worried that your balance sheet is 'debasing' the currency.
EDIT: And just to be very clear to the 2 people who read this comment, maintaining a balance sheet is still market support b/c you still buy treasuries on the open market to offset the principle of your existing treasuries that reach maturity. So stopping the growth of the balance sheet just means you're not accelerating support. Tapering is the thing that you do if you're worried that your balance sheet is 'debasing' the currency.