Because there is only one (no economies of scale), because it is large, because it is insanely precise, because it has to work the first time, because it is traveling very far, because it has to withstand the stress of launch, be 'foldable' (far easier said than done), because they encountered a number of problems while testing the various subsystems requiring re-design/re-manufacture, because it runs a delta-t of 290 some degrees celsius across a two meter distance and so on.

This is probably mankind's most amazing technological achievements bar none.

When you put it like this, 9 billion sounds like a good deal :)

Also, consider how many (presumably) well paid expert engineers are scientists have been working on this since 1996.

A combination of space engineering costs being habitually lied about in proposals and the conventional space industry being about half science, half jobs program.

Frankly I wish someone would do an article on the "long tail" possibilities for Webb. Can it do a warm mission after its refrigerant runs out like Spitzer? Does it have low-bandwidth long range communications for when it runs out of fuel and falls out of L2? I feel like we're just supposed to be ok with a ten billion dollar telescope only lasting a decade.

Only one instrument the MIRI needs active cooling. Rest is all passive cooling. It has a closed loop cooling system so it will not run out of refrigerant. Once it runs out of fuel it will eventually drift out the L2 point and might lose orientation. They feel they can stretch the mission up to 20 years with the saved fuel. There is a port for refueling the craft in the future but the technology needs to built.

Why would it lose orientation? It has reaction wheels.

The simple answer is that you eventually have to do thruster burns to dump momentum from reaction wheels; they can't keep absorbing momentum forever. It also needs the fuel to adjust it's orbit every so often and keep it on station.

The question is "is it worth it?".

New scientific instruments usually come with a bunch of discoveries, because we are now able to see what we couldn't see before, but there are diminishing returns, because we run out of interesting things to look at.

Some platforms can be upgraded, like the ISS, ground based telescopes, and Hubble in 2009, so that new science can be done, but it is not the case for JWST. So I suspect that there will not be much left to do with it after 10 years, especially if it is not fully operational. It will not be completely useless, but I expect money to be better spent making a new instrument, with better technology and guided by the latest discoveries, than trying to extend the life of an aging instrument that has given most of what it could give.

My understanding is that most off the money went into designing and extensively testing one off components that work reliable in extreme environments.

Engineers are expensive

Hmmh, I'd think since much of JWST is i) one-off and ii) state-of-the-art, a lot of it was actually designed by PhD students, which are cheap.