Their strategy has been to focus on conversion optimization. That means the chance of a user ending up booking a hotel given that they visit Booking.com’s site is much higher than other hotel booking services. The result is that they make significantly higher revenue per visit than their competition, which then allows them to outbid all of them in ad spending, because a click is worth more to them than anyone else. I heard they were the #1 advertiser by revenue on Google Adwords at one point.
The fact that they are based in Amsterdam, they are owned by a company whose brand is a smaller internet product, and most of their traffic comes from ad spending, makes people forget about them, but they are one of the biggest and most profitable internet companies.