the fleets were bought with different purposes in mind.
The ORIGINAL KC-135 fleet purchase was to support the SIOP. Period.
The KC-10 was bought to support mobility deployments of fighter squadrons.
The current use of the tanker fleet is obviously much wider than the original intent but to answer your question - that's why.
That fleet of tankers is one of those key enablers that gives not just the USAF, but the real warriors in Naval Aviation our global reach and mobility.
To replace that fleet will actually take three increments. Since much of the KC135 fleet were newer aircraft and upgraded to the R configuration, that part can wait. The initial increment was to replace about (I think) 2/3 of the fleet of older 135's with follow on competitions for the KC-10 and then the KC-135R replacements.
As an aquisition decision - this is utter folly.
I understand the reluctance of this administration to bequeth a done deal on the next administration, regardless of which one it is. Of note, McCain has been very interested and involved in the tanker problems and was instrumental is forcing the open competition rather than the kick-back lease deal. So sticking either him or The Audacious One with a monstrous hot potato(e) would be politically sensitive to say the least.
However, after a horrendously costly competition and incredible level of effort by the DoD, AF and the competing teams - its all back to square one and kick the can down the road. sad......
As a defense contractor - I know the costs of competing, and having supported on the defense side some source selection processes, that is an incredible amount of work.
It would not surprise me to see various members of these teams, or even an entire team, bail out rather than go through that hell yet again.
This does not say much for our aquisition competence.
And why, before someone raises the obvious question, is it this hard to go through the aquisition process? Because that is how Congress set it up when they wrote the laws that we must follow.