Why do you think Anna Avalon did not reach out for her husband while he was falling in "The Leap"?

Anna Avalon chose life rather than death as she fell through the air.
Anna always contended that a person can think of many things as he/ she is in the act of falling. As lightning strikes the big top, Anna Avalon realizes she cannot save her tumbling husband. Instead, she turns in the air and twists her body toward the heavily braided wire, saving herself and her unborn child.
Unfortunately, Anna hemorrhages after her fall from the trapeze, and her baby is stillborn. She does regain her health, though, and her broken arm heals well, thanks to the attentive care of her physician, who falls in love with her. They marry, and their only child is the narrator, who declares that her mother's words about being able to think during the brief time that one is falling are absolutely veracious.
Louise Eldrich's narrative soars on its own as the protagonist leaps from one daring act of love to the next, proving the strength and agility of the human spirit that fortifies its existence with the love necessary to make the leap to the next challenge. For, whenever one is in the air "there is always time to think." And, with the impetus of love, there is always time to change the course of one's life.   

In "The Leap," Harry Avalon is killed when a freak bolt of lightning hits the main pole of the tent, causing him to fall to the ground. When describing this tragic accident, the narrator mentions that her mother, Anna, could have possibly saved his life:

"As he swept past her on the wrong side, she could have grasped his ankle, the toe-end of his tights, and gone down clutching him."

But Anna did not do this. Instead, she changed the direction of her body and clung on to a "heavy wire" which burned her hands but enabled her to climb safely to the ground.
As Anna was seven months pregnant at the time of the performance, it is likely that saving her unborn child was her priority. In that split second, she realised that she could not save her husband and her child so she chose the latter, perhaps driven by maternal instinct.
In fact, Anna's desire to protect her children is one of her defining characteristics, as we see later when she rescues the narrator from a burning house. It is, therefore, part of her nature to put the lives of her children before anyone else, even her own husband.