The magazine New Scientist just put up a very interesting article on abandoned space projects.
Take note of when most of the projects were canceled; either in 1993 or 2001-2002--basically when a new political party took over the White House. I suspect that most of them weren't canceled because they were necessarily bad ideas or too costly (what space project doesn't have cost overruns?), but simply because the new administration wanted to kill the projects of the previous one out of politically-motivated spite.
As I've stated in previous entries here, we have to stop this political game of tit-for-tat every eight years with our space program if we ever want to accomplish any long range goals, such as returning to the Moon or going to Mars. Space is just to vast and difficult an undertaking for such short-term petty-minded tactics to get anywhere.