Because there's a rather finite amount of money spent on air travel which means that the valuation of any one company in that sector will be well within Google's means?
http://www.itasoftware.com/news-events/press-release.html?id.... Note that the round included Sequoia who are known for caring only about big markets.
From what I hear it worked technically, but failed commercially due to the badly timed recession. It seems like a fair bet that Google's offer wasn't putting a lot of value on the reservation system. Even if it could be made to work, it's not the kind of a business that Google is usually in.