I can see a couple factors: * To be useful to science, the telescope needs a huge diameter which is more than can be launched so they had to make a folding tel

I can see a couple factors: * To be useful to science, the telescope needs a huge diameter which is more than can be launched so they had to make a folding telescope that can be later unfolded. * We only get a single chance for success so testing becomes critical. Lot of testing was needed to make sure everything would work out in the end. * The technology needed was pushing the limits of what we are capable of so lot of R&D needed.