KRSchultz is basically correct, please don't downvote him. The last Hubble repair mission went way beyond the Hubble's expected service capabilities, which was an amazing accomplishment.
However, it's vastly more economically sound and safer to bring an aging device down in controlled descent and send up a brand new one with all the latest bells and whistles than it is to send up the Shuttle with a crew and do space walks for a repair and upgrade mission.
The shuttle weights 4.5 million pounds not including cargo. The Hubble telescope weighs 24,500 pounds. Most of the cost is getting things into orbit, and when building devices the cost of a second spare is insignificant compared to the cost of the first one. Obviously sending a 4.5 million pound shuttle as payload with crew into orbit costs considerably more and also destroys more of the ozone layer than sending a 24,500 lb payload.
