Kayak and Orbitz are using matrix software by ITA, which is in my opinion the best airfare-searching engine at the moment. Notably, Delta's partners: Air France, Alitalia, Alaska are in the list of ITA Software customers. So are United and American. http://itasoftware.com/about/customers.html
Delta managed to write their own engine, and it is, apparently, not as good as ITA's. Perhaps somebody in Delta IT management team can swallow their pride and purchase leading back-end engine vs. attempting to catch-up to it with their own code.