No one can really explain it fully, because it is a behind-the-scenes selection by HP staff.
All you can do is strive to write high-quality hubs, with proper grammar and spelling. The topics should be "evergreen," meaning of interest to a wide range of audiences all year round.
Well-done photos, either your own, (or legally used and properly credited) are also a great idea.
Beyond that, there is a certain amount of mystery and secrecy. ;)
Check out the Hub Pages blog post on the matter for further details.