They were destroyed by the taliban
edit: Oh Sorry… they were built by/ it is believed to have been built by the Kushans (ancient india)
Bamiyan was along the Silk Route, and thus many Indian and Chinese Buddhist monks were there. The statues were most likely the work of these monks.
The List of World Heritage in Danger: 2003
Cultural Landscape and Archaeological Remains of the Bamiyan Valley.
The Buddha statues and the cave art in Bamiyan Valley are an outstanding representation of the Gandharan school in Buddhist art in the Central Asian region.
The site is also testimony to the tragic destruction by the Taliban of the two standing Buddha statues, which shook the world in March 2001.