Because she was slim again 3 month after delivery. There is no question, that she looked fabulous. The question is how she could do it.  There are mainly two guesses about how she  lost weight so fast.
First is the dieting.  Angelina Jolie is 5’8”, and her estimated weight now is 112 so BMI is 17. It is well below  19.5 as a lower limit.  Second is the tummy tuck, which is abdominal plastic procedure.  Tummy tuck removes the excess of the fat and  skin, which makes the belly look trim and slim.
In my opinion the dieting is the most plausible explanation, then tummy tuck.  The reason: she had already had fast weight loss in 2007 , so she knows how to do it. 
I do not consider exercise as a reason for significant weigh loss, because the energy spent during workout is relatively small in comparison with amount of calories consumed. 
A tummy tuck may be risky: remember what happened with Mrs. Donda West, the mother of Kanye West? As all we know, she died after tummy tuck, breast reduction and  liposuction. May be we should rethink our attitude to surgical cosmetic procedures, especially if they are risky.  I understand how to go to surgery for a problem, which  cannot be solved by other means.  But to risk your life for vanity?
How can we remove the excess of fat from the belly, if it is related to aging and not related to pregnancy?
Diet:  no processed food, portion control, minimum cooking, just for safety.
Hormone correction.
The main reason why women gain weight especially  after menopause is hormone levels change. So bioidentical hormone replacement therapy may help. If you have an idea how to bring weight down fast please let me know.