With so many people in the world having different tastes, how do cover designers arrive at covers that they feel will appeal to the most people?
Art and design, like genre trends, go in and out of fashion. A good designer keeps current on what is hot and what is not, and hopefully creates a cover that is on the leading edge of coolness. Sometimes it works; sometimes it doesn't.
A publishing house will generally have a designer do several mock-ups in varying styles, then run those mock-ups by a group of employees—the idea being that the cover with the most appeal to the staff will also have the most appeal to the general public. Again, sometimes it works; sometimes it doesn't.