It seems as though they have a very sustainable advantage. Getting all the hotels connected is expensive. Because they have such a large number of hotels in place with a good revenue share they probably have some of the highest conversion rates in the industry. The higher conversion rate and deep pockets allow them to buy demand and brand awareness. Very hard to see how someone would enter this market and try to compete with them. I imagine they also have exclusive deals in place with some hotels making it even harder.

It's not impossible though:

1. One of the other big tech players could decide to enter this market. 2. Alternatively a site with huge demand could start to feature hotels (airbnb for instance). 3. A technology shift or a shift in the right advertising strategies to create demand could leave room for startups to enter this market.