Paperbacks are generally cheaper than hardcovers. When the editor and writer save money in the production process, the savings are passed on to the consumer.
Being able to easily take a book with you is a huge convenience. Paperbacks are free of the weight and bulkiness of a hardcover.
Many people like to have a nice-looking bookshelf. Hardback books are more likely to look neat and uniform as apposed to a wrinkled paperback.
A hardcover is likely to age more gracefully. Paperbacks tear, wrinkle and stain much more easily.
Authors want to make an impression with their book release, so hardcover is the first choice when printing. So if you prefer a paperback, you may have a long wait.