why couldn't they cure what the mother was sick with?

No, she had congestive heart failure. She was scheduled for a heart transplant but that fell through and she went downhill from there.

If the book is true to like, ovarian cancer spreads quickly and has a high fatality rate.

~May peace rain down from Heaven~

God wanted her to die - He had a pair of shoes to shift.

"The hour is come but not the man"

Maggie's immune system was failing and her blood type was hard to match. Difficult to find another match.

Can you fly this plane?
Surely you can't be serious.
I am serious, and don't call me Shirley

In the Book it was Ovarian Cancer, In the movie it was Heart failure. Always wondered why they made that change.

You might as well rephrase your question with "why couldn't they perform magic?"