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.