We learn a lot about this in the Apprenticeship Program. The reasons have to do with the fact that what makes a hub popular for HubPages readers is simply very different from what makes a hub visible on Google.
To be found on Google, you want good organic Search Engine Optimization (SEO), which means a title, keywords, summary, and main headings in your hubs that contain phrases that Google's analysis tools will match to search phrases that are often used.
None of that matters on HubPages, where your followers will see you, and people searching the category for your hub will see you.
Also, readers on Google are looking for answers to questions. Readers on HubPages are also writers. They are looking for stimulation, interesting ideas, and community.
A good analogy might be this: Articles that do well on Google are like the books that make popular best-seller lists: catchy titles and a good read. Articles that are read and appreciated by Hubbers are like books that win literary awards. More depth, less catchy.