Its EXPENSIVE and all the shuttles were getting too old and dangerous and they decided they could do most of what needs doing using modern rockets in a more cost effective manner. Also private sector development of space technogy has reached a point that anything that needs a larger space craft can be contracted out to save money as well.
The space shuttles had reached their age limit. Originally the plan was to decommission them even earlier. It was just the lack of a alternative that kept them going, since the ISS needed regular supplies. The real question is, why did they not develop a successor early enough.
Congress kept asking for new designs to replace the shuttles, then refused to fund them because they were 'too expensive'. It's called kicking the can down the road, the idea is that you can claim you support space exploration, but you can't be blamed for making a wrong decision, if you never decide.
The same thing happened with the M 1 Abrams tank, which kept getting it's funding cut because it was too expensive. Left to their own judgement, Congress would have our military equipped with the leftovers from WW2, because new equipment would be untried, too radical, unproven, and too expensive.
So, we get our rides into space from the Russians, and pretty soon from the Chinese. Woo, USA, Number One!