Just speculation on my part, but with 60 plus percent of all searches (at least in North America), Google is the 800 lb gorrilla that all the other chimps aspire to be.
They have the biggest inventory of "eyeballs" via either their search engine results and ad network, and advertisers are willing to may more for access to all these i-balls.
It may cost more per ad, but the advertisers will be reaching - or at least have a better chance of reaching - a larger, more targeted audience and thus getting a bigger bang for their buck.