To make broad generalizations, the point of quantitative easing is to increase the money supply. You do this by having more money in banks like Goldman Sachs (and of course there are many other banks that the Fed will be buying treasuries from).
The idea behind this is that once banks have more cash on hand, they will be more likely to loan that money out to other businesses and people--and voila, suddenly the economy has more money in the system.
If the Fed buys bonds from the Treasury, however, none of this happens. Whereas before the Fed was printing money and injecting it into the economy, selling to the Treasury is roughly equivalent is printing money and having the government hold on to it. And of course, having the government hold on to that money doesn't do the economy any good in term of monetary stimulus.
The idea behind this is that once banks have more cash on hand, they will be more likely to loan that money out to other businesses and people--and voila, suddenly the economy has more money in the system.
If the Fed buys bonds from the Treasury, however, none of this happens. Whereas before the Fed was printing money and injecting it into the economy, selling to the Treasury is roughly equivalent is printing money and having the government hold on to it. And of course, having the government hold on to that money doesn't do the economy any good in term of monetary stimulus.