Because you do not need to use as high a price of return when there's high demand for MBS, home mortgage rates are less than they would be if the Fed weren't doing all that acquiring. The FOMC has straight control over something called the government funds rate. This is the rate at which financial institutions borrow from each other when they need funds overnight. When rates of interest are higher, even more people will want to get bonds-- why do not greater rate of interest press bond costs up?