For example, let's say you run a bookstore. Your customers will come to buy books, but if you simply set up credit card processing and call it a day, you're missing opportunities. Talk to customers and find out how they want to find and purchase books. Paying attention to users' goals can open up new directions for your business, not just new paths through your site. They're successful because they listen to customers.
Luck has very little to do with it.
Another important thing to consider is that many potential customers don't make it to the checkout. Chances are, they have some sort of need that has gone unanswered. It might be that they have concerns about security that have not been addressed. It might be that they need to search for a particular author, but your search function doesn't support that. It might be that some technology in use on the site crashes their browser.