Short chapters are a gift to readers who may not have the time or stamina to handle 70 pages at a sitting.
For a young reader, reaching goals is important. As their eyelids get heavy and their mind starts to wander into the dreamlands, they struggle to read just a little bit further. If only they can make it to the end of the current chapter, then they will have accomplished something.
By keeping the end of each chapter within reach of even the most tired juvenille reader, an author facilitates little victories in the lives of those same young readers. Those victories, as much as marvelous characters and unexpected plot twists, can fertilize a budding love of reading...
And that love of reading, in time, can lead to increased revenues for the author, when subsequent sequels and new series are released.