Why did Mary Pope Osborne write Magic Tree House?

She is sharing her imagination. Initially, she was approached by Random House to create a series about some children who went to different places and times. Not wanting to be tied down the same old story line she balked at the idea, but with some encouragement and six attempts at books you have never read she and her husband came upon a dilapidated old tree house during their travels together and the rest is history as they say.