At the beginning of the Obama Administration, NASA was redirected to focus on missions to mars, asteroids, and commercial space flight;
This forced NASA to cancel a multitude of projects, including the restart of the X-33 program, which was supposed to replace the soon-to-be retired space shuttles.
The X-33 was an single stage to orbit spacecraft, making it far more durable and cheaper to launch large amounts of cargo into space.
It was initially suspended in 2001 because some components needed, had not been developed yet. Since 2001 the technology needed has been developed, and would have been implemented in the construction on the X-33, if it hadn't been completely canceled in 2009.
The X-33 would allow greater access to space, and allow NASA to make the most of the limited budget.