The perennial and wrong "Obama killed the shuttle", yet again
The shuttle program was irreversibly shut down during the 43rd President (George W Bush), for entirely sensible reasons of safety and cost. Don't have to believe me, believe the then Space Shuttle Program Manager, writing in 2008:
Starting four years ago, the shuttle program in its various projects made "lifetime buys". That is, we bought enough piece parts to fly all the flights on the manifest plus a prudent margin of reserves. Then we started sending out termination letters. About two years ago, we terminated 95% of the vendors for parts for the external tank project, for example...
You might think that simple things like bolts and screws, wire, filters, and gaskets could be bought off the shelf some where, but that thinking would merely prove how little you know about the shuttle. The huge majority of supplies, consumable items, maintenance items, they are all specially made with unique and stringent processes and standards...
We started shutting down the shuttle four years ago. That horse has left the barn.
As a taxi to the ISS the shuttle program was replaced by "Commercial Orbital Transport Services", with Russian manned orbital transport being a stopgap until US-based crew transport starts.
There is no replacement for the shuttle as a Hubble service mechanism: the general opinion is that it would be better value to develop and launch a replacement than to spend the resources to build a suitable servicing craft.