You have to remember that tankers are an essential and valuable asset, and
the current ones are based on 707's. They are getting long in the tooth,
considering the hours and hard use they have endured.

Even refurbishment in a depot can not always resurrect an old and tired
airframe. Simply put, the Air Force needs newer tanker assets, and it would
be simpler to modify an existing airframe rather than go through a clean
sheet design.