I don't stay on top of things like I should, but IIRC, a year or two ago they said the reason they were retiring the Delta II is because the Delta IV will take over its role. While the Delta IV is a nice vehicle, it is very expensive. In fact, if not for Congress it would be gone and its heavier payloads would all go to the Atlas V, which is much cheaper but uses Russian engines. I think they still have plans to kill off the Delta IV when the Vulcan Centaur comes online.

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