The Fed might prefer to wait to adjust its portfolio so it can focus on the short end of the yield curve, where it is more familiar with the markets and historically has had more control. Policy makers may also be concerned about the outlook for long-term rates. An important justification for QE and the emergency liquidity facilities that preceded it was to circumnavigate the troubled portions of the financial system and lower rates at the longer-term maturity points that drive borrowing costs on mortgages and other types of credit with direct relevance to the real economy.
The Fed’s large holdings of long-term securities have helped hold down long-term rates by depressing term premiums. According to estimates by the New York Fed, the average term premium for the 10-year Treasury note in November was 13 bps, compared with an average of 164 bps since June 1961. Normalizing the balance sheet any faster could drive long-end rates higher than the Fed would like.