Being completely ignorant of ongoing NASA or private venture space projects, if the Hubble telescope is so useful and oversubscribed (not to mention old) wouldn't it make sense to launch a few more Hubbles sometime soon?
There are a fair few other space observatories according to Wikipedia, however I suspect most of them have special missions. As I understand HST is fairly versatile with a wide range of equipment. It is also estimated to cost over $10 billion, so sending a few more up would each cost over half of NASAs annual budget :D
Does 10 billion for one mean that it will be 20 billion for two? Once it is designed, and the teams are in place to assemble it and run the thing, economies of scale should kick in. Would be nice if most of that 10 billion was a sunk cost, and for 20 or 30, we could send up 50 of these things.
Money has already been mentioned, so has James Webb space telescope. But also worth mentioning is that advances in optics (active and adaptive optics, better mirror manufacturing processes) and image processing has made earth-based telescope a viable alternative for space-based telescopes.
It's a lot cheaper to build (and fix!) telescopes on the surface of the earth (in a few select locations like the Atacama desert, the Canary islands and mountains on Hawaii) and the best of current telescopes can deliver images with similar quality to what the Hubble space telescope can produce.
The mirror and spacecraft are old, but all of the instruments are relatively new, having been put into the spacecraft during the last servicing mission[0].
There's no money for it and there probably isn't enough enthusiasm for it in Congress. The US economy is $15 trillion dollars. We spend $40 billion on pet food to give you and example of the money the US consumer has.
Perhaps, someday someone will figure out how to tap into that pile of cash for Big Science.
