Why did the Treasury and Fed let Lehman fail but rescue Bear Stearns, Fannie Mae, Freddie Mac, and A.I.G.? IG

I have been asked this questions so many times during the past week. This is the closest
I can find for an answer if any. Please read the interview and it definitely touch upon
many interesting areas. 

Diamond and Kashyap on the Recent Financial Upheavals - Freakonomics - Opinion - New York Times Blog

3) Why did the Treasury and Fed let Lehman fail but rescue Bear Stearns, Fannie Mae, Freddie Mac, and A.I.G.?