Why is the Space Shuttle's External Tank fuelled through the Orbiter's main engine plumbing system?

Question

This answer states that Space Shuttle's External Tank (ET) was fuelled (this process is known as Tanking) through the Orbiter's main engine plumbing system with the Tail Service Masts (TSM). One delivered LH₂ and the other delivered LOX.

For reference, here's a picture of the TSM fuelling Space Shuttle Atlantis.

However why not just directly fuel the ET instead of the fuel and oxidizer going through the orbiter. It seems like the orbiter's fuel plumbing system would be a lot more complex with fuel going from the TSM, into the orbiter, and then into the ET. Not only that, but the LOX had to be pumped up to where the LOX tank was located (in the upper half of the ET) whereas if it was directly fuelled into the ET, the pipes can be pumped upward in the tower possibly saving weight on the ET by not having a LOX feed-line.

Question: So what are the advantages or reasons that got NASA to fuel the ET through the orbiter instead of directly fuelling the ET?

Answer 1

I don't have a great reference for this, but it was to reduce cost on the throw-away External Tank.

By using the same interface into the Orbiter used to supply propellants to the main engines, the cost and complexity of adding a dedicated loading interface to the tank was avoided.

It was not a tremendous complexity hit to the Orbiter Main Propulsion system plumbing. Basically "tees" and valves were added which interfaced the feedlines running from the tank to the engines, to the umbilical disconnects that interfaced with the Tail Service Masts.

(From the Ascent Pocket Checklist, fill lines highlighted)

As the only disposable major element of the system, there was great emphasis placed on cost of the tank. This is clear from even a skim of Lessons Learned From Space Shuttle External Tank Development - A Technical History of the External Tank - although the specific case of lacking a dedicated loading interface is not addressed in the document.

Answer 2

How are you going to feed LOX to the main engines without a feed line? Whether the feed line is on the outside or inside through the LH2 tank, you still need to have a means to transfer LOX from the LOX tank to the main engines, you cannot avoid it. Using the main engines feeding lines to fill/drain the external tank is simpler, therefore practical and cheaper. You don't need to have extra hardware like separate LH2 and LOX feed/drain valves and umbilical plates on the external tank, as well as additional launch pad hardware like service arms for that same purpose. It makes the external tank more expensive as well as the launch pad. Not to mention added complexity to the launch operations.