I have read many times that the Delta IV is more expensive to launch than the Atlas V. Outside of the Delta IV Heavy configuration, which is in a class by itself for certain payloads and missions, are there other reasons a customer might choose to launch on a Delta instead of an Atlas? There must be, since there have been a number of Delta IV launches in the 15 or so years since the Delta IV family was introduced. But I can't figure it out.
Since 2004, it's been policy of the US government to assure there are at least two launcher families available for launching "national security payloads" (e.g. military surveillance and communication satellites). Prior to the entry of Antares and Falcon 9 in the market, this pretty much meant Atlas and Delta, therefore USAF was required to make the necessary orders to keep both rockets in production.