Why? Optimism, high energy, succinct but compelling character descriptions.
Hubbard wrote two dozen novels and hundreds of short stories. The bulk of this output came in just a seven year span in the 1930s.
Sometimes it seem that the only place for optimism in fiction is middle grade books (8-12 yrs), romance, and westerns from dead writers. The trend is for the ambiguous ending. Or the outright pessimistic.
Many great books have been written with depressing conclusions. But perhaps the pendulum has swung too far in that direction.
Hubbard made his name in the pulp era of the thirties. With no TV or internet, people went to the drug store and purchased entertainment in the form of pulp magazines or dime novels. They were cheaply made. They were constructed to deliver thrills to a depression era population. Life could be better. You too could be a hero.
Hubbard was the superstar of this era. He pounded out massive word count, published hundreds of stories. Sometimes a pulp mag would be filled with just his stories and they would assign each story to a different pen name to make it look like a number of authors had contributed. Like many great writers, he had such varied life experience that it was easy for him to switch from a story about glider pilots to espionage in pre-WW2 China.
Some writers spend pages on character development. Hubbard had a knack for writing a line or two that made you feel like you knew the character.
For a light introduction to his works, read, The Hell Job Series. Battlefield Earth is one of my favorite Sci-Fi books.