Why did Columbia’s planned flight manifest have a five-year gap with absolutely nothing in it between 2004 and 2009?

Question

Looking at the pre-loss-of-OV-102 plans for then-future shuttle flights (at least according to the document found and scanned by @OrganicMarble and posted as part of this answer, which was drawn up [the document, not the post] while Columbia was in orbit on STS-107), one quickly notices something odd about Columbia’s planned flight manifest. Unlike Discovery, Atlantis, and Endeavour, which were booked solid with a rapid-fire sequence of ISS-component-and/or-crew-delivery flights out through 2010, Columbia had only three flights in the pipeline:

• A single ISS servicing mission (STS-118) in November 2003 (the only time the oldest orbiter was ever planned to visit the ISS);¹
• A Hubble servicing mission (STS-123) in November 2004;²
• and, finally, five years and one day later, in November 2009, STS-144, a mission to retrieve the HST and bring it back to Earth (almost certainly to go on display in the Smithsonian).

The lack, on Columbia’s part, of the rapid-fire ISS flights which made up the entirety of the other three orbiters’ planned flight manifests is understandable, given that Columbia was the least-suitable orbiter, by far, for ISS flights (being the oldest, and, thus, heaviest, and, thus, most-marginal-for-high-inclination-flights-[such-as-those-to-the-ISS] orbiter, and the only one to still have its airlock mounted in the orbiter’s middeck, rather than being equipped with the payload-bay-mounted airlock-and-docking-adapter assembly needed for docking with the ISS), but I’m having trouble seeing why Columbia, not being encumbered with a heavy ISS flight schedule, couldn’t launch other payloads in the meantime,³ or do more SCIENCE!!! missions,⁴ unless they needed four or five years to modify Columbia for STS-144, which stretches the bounds of credibility well past the breaking point.

So why the massive gap?

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¹: STS-118 was eventually flown, much later, by Endeavour in August 2007.

²: Following the loss of Columbia, the fifth HST servicing mission was initially cancelled, but was later resurrected, and flown by Atlantis in May 2005, as STS-125.

³: You know, like it was already doing, and like all the other orbiters had also been doing prior to being put on ISS-assembly duty...

⁴: Like how Columbia had been the Spacelab workhorse orbiter in the 1990s, until NASA suddenly decided to retire the Spacelab modules.

Answer 1

When I read this question, my gut reaction was

"Because there was no money for anything else but the ISS."

I still think this is correct, but may be hard to substantiate. I looked for evidence in the Columbia Accident Investigation Board (CAIB) report - they went into the NASA budget situation prior to the accident in some detail. There is nothing that explicitly says this that I could find, but there is some supporting information.

• STS-107 itself, being a non-ISS mission, was considered low priority and had to have special funds added to the budget to support it.

In October 1998, the Veterans Affairs and Housing and Urban Development and Independent Agencies Appropriations Conference Report expressed Congressʼ concern about the lack of Shuttle- based science missions in Fiscal Year 1999, and added \$15 million to NASAʼs budget for STS-107. The following year the Conference Report reserved \$40 million for a second science mission. NASA cancelled the second science mission in October 2002 and used the money for STS-107....

After 13 delays over two years, due mainly to other missions taking priority, Columbia was launched on January 16, 2003 (see Figure 2.1-1).

(CAIB, Volume I, p. 27-28, emphasis mine)

• NASA's budget, in real dollars, was dropping precipitously in the era prior to STS-107 and the ISS was eating the Shuttle program's lunch.

The search for cost reductions led top NASA leaders over the past decade to downsize the Shuttle workforce, outsource various Shuttle Program responsibilities – including safety oversight – and consider eventual privatization of the Space Shuttle Program. The programʼs budget was reduced by 40 percent in purchasing power over the past decade and repeatedly raided to make up for Space Station cost overruns, even as the Program maintained a launch schedule in which the Shuttle, a developmental vehicle, was used in an operational mode. In addition, the uncertainty of top policymakers in the White House, Congress, and NASA as to how long the Shuttle would fly before being replaced resulted in the delay of upgrades needed to make the Shuttle safer and to extend its service life.

(CAIB, Part 2, Chapter 5, p. 99, emphasis mine)

• The new NASA administrator was chosen not for technical prowess but for his budgetary acumen. One of his first major decisions (in fact, he announced this plan even before he was chosen as administrator) was to cancel the US "lifeboat" vehicle and the Hab module for the ISS, limiting the US staffing to three crewpersons and severely restricting the science potential of the station.

During congressional testimony in May of 2001, Sean OʼKeefe, who was then Deputy Director of the White House Office of Management and Budget, presented the Administrationʼs plan to bring International Space Station costs under control. The plan outlined a reduction in assembly and logistics flights to reach “core complete” configuration from 36 to 30. It also recommended redirecting about $1 billion in funding by canceling U.S. elements not yet completed, such as the habitation module and the X-38 Crew Return Vehicle. The X-38 would have allowed emergency evacuation and landing capability for a seven-member station crew. Without it, the crew was limited to three, the number that could fit into a Russian Soyuz crew rescue vehicle.

....

A week later, Sean OʼKeefe was nominated by President Bush as the new NASA Administrator.

(CAIB, Part 2, Chapter 5, p. 117)

• The 'temper of the times' can be judged by the fact that the only reason Columbia was scheduled to fly to the ISS for that one mission was a desperate attempt to maintain the schedule. The external airlock would have had to be removed later for the planned Hubble retrieval mission.

To meet the new flight schedule, in 2002 NASA revised its Shuttle manifest, calling for a docking adapter to be installed in Columbia after the STS-107 mission so that it could make an October 2003 flight to the International Space Station. Columbia was not optimal for Station flights – the Orbiter could not carry enough payload – but it was assigned to this flight because Discovery was scheduled for 18 months of major maintenance.

(CAIB, Part 2, Chapter 5, p. 117, emphasis mine)

tl;dr the NASA budget environment at the time was bad, and all financial resources were focused on completion of the ISS. There was no upper management interest in or money for funding standalone Shuttle missions.