And since you are going to spend so much money to launch it, you also add extra hardware to do other type of observations, so the cost of the telescope balloons upward but you are get more bang for the buck.

If on the other hand you have cheap launches then you can send up the minimum telescope needed since you can always send up a better one or even a very different one if you later discover the need.

Today because of high costs, we need to design the scopes to meet all the possible needs we can think of and afford, result very expensive space scopes.