Why was the Space Shuttle Orbiter's in-orbit time limited?

Question

I remember reading/watching that the Shuttle had a limited time to stay in orbit. And that's why Soyuzes were the only Assured Crew Return Vehicles (ACRV) for the International Space Station until Dragon 2 came along.

Besides the why, if possible, I'm also interested in the design choice, i.e. why was an alternative choice not favorable for the Shuttle?

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Here's the research I did:

All I could find from Wikipedia is:

Most missions involved staying in orbit several days to two weeks, although longer missions were possible with the Extended Duration Orbiter pallet.

— Wikipedia: Space Shuttle § In orbit

The vehicle's own article (Space Shuttle orbiter) does not mention that pallet, or as far as I searched the article, the duration limitation.

My working hypothesis is that Soyuz has solar panels, while the Shuttle relies on fuel cells.

I wondered why not power down the Shuttle, or put it in a power-saving mode until needed. Then I thought maybe the hydrogen leaks, so I searched and found a NASA page on the fuel cells, which wasn't helpful, but a further reading link, and thanks to Web Archive, I arrived at a more detailed NASA page. From which:

During normal fuel cell operation, the reactants are present in a closed-loop system and are 100 percent consumed in the production of electricity.

100% hints at no leaks in normal operation, which was confirmed further down the page:

Reactant consumption is directly related to the electrical current produced: if there are no internal or external loads on the fuel cell, no reactants will be used. Because of this direct proportion, leaks may be detected by comparing reactant consumption and current produced. An appreciable amount of excess reactants used indicates a probable leak.

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I googled for the reason, but the first couple of pages were not helpful. (And strangely, no results linking here.) A recent find I came across which prompted this question is that even though Russia was not of the four countries involved in Space Station Freedom, Soyuzes were still considered for the ACRV role.

Answer 1

I found the answer in the EGIL Console Handbook (not online). This was the flight control position responsible for the Orbiter's electrical system.

The Station to Shuttle Power Transfer System (SSPTS) was developed in order to increase the docked duration from ~ 6-8 docked days to ~ 9-12 docked days. The upper limit of 12 docked days (16+2 day mission) is due to orbiter certification limits (in order to surpass a 16+2 day mission items such as MMOD, S-Band Quad Switch, OMS/RCS Thermal Constraints, Tire Pressure Maintenance, etc. would need to be evaluated)

In other words, even with SSPTS which allowed the Orbiter to draw power from the ISS and therefore reduce its use of fuel cell reactants by ~half, costly studies would have to be done to allow longer docked missions.

Also, at least in the ISS construction phase, Orbiters were not available to be left hanging out at the ISS for months, so there was not much motivation to do those studies.

Acronymology:

• MMOD: MicroMeteroid and Orbital Debris
• OMS: Orbital Maneuvering System
• RCS: Reaction Control System