Money pure and simple; same reason for back yard breeders. Some people got into puppy mills because their families have been farming for years, and dogs are just another livestock. A petition won't stop puppy mills. Sign up with the Human Society
Stop Puppy Mills, and receive literature that has important laws that are in legislation. Join the YAP club here, if you haven't already,
http://www.yorkietalk.com/forums/group.php?groupid=61, and help keep other members informed of the laws. When you find new legislation that pertains to a certain state, post a thread so that those members who live in that state can write to their representatives. Legislation is important in stopping puppy mills, it may not stop large kennel breeding, but it will raise the conditions in which the animals are being kept.
Education is an extremely important, when people ask for advice on choosing a breeder, always recommend that they never buy from a Website, Websites are useful in informing people about your breeding program, but should never be used as the method in which to choose your puppy. Advise people to never purchase from a breeder whose home hasn't been checked out to ensure that dogs are kept, in suitable conditions. Inform buyers not to be tempted to rescue a dog from a bad breeder because another puppy will just take it's place, and the parents will continue to live in horrible conditions. People who find dogs kept in unlawful conditions should report this to the proper authorities, and not be tempted to rescue. Everyone expects puppymillers to make large changes in their kennels and breeding programs, but individuals so often don't want to make the smallest change, such as not buying from a website or pet store. This would have a huge impact, but most individuals have excuses, and believe that the breeder on the website they've chosen is different, and they are easily influenced by cute pictures of the puppy or breeder.