The mass-market paperback, for one, is too expensive.
Science fiction readers probably have the gene for novelty, and seem to enjoy a cascade of invention as much as a writer enjoys providing one.
Try to meet as many authors, agents, and editors as you can.
How long it takes to write a book depends on its length.
It's a tough job to tell a story when the audience already knows the ending, and the ending is bleak.
Even if you only want to write science fiction, you should also read mysteries, poetry, mainstream literature, history, biography, philosophy, and science.