Why are you asking Why?
Do you expect an answer?
I'm gonna guess you won't get any.
My point is, constantly asking yourself Why will do nothing except further confuse and frustrate you searching for an answer that simply doesn't exist..
Its kind of like asking for the meaning of Life..I'm sure there is an answer, but we'll never truely know.
Spend time with your mom, show her smiles and positive thinking. Right now above all else she needs support and people to believe in her that she can beat this. Help her laugh and smile, as laughter is by far the best thing you can do to keep her in good spirits
Take care, and I hope everything works out ok