http://en.wikipedia.org/wiki/Buddhas_of_Bamyan
Ego at its worst destroyed them.
However, their creation was probably done out of devotion by the Kushan artists.
Quite possibly their inspiration was Egyptian statues and Greco-Roman sculpture.
In contrast with traditionally Indian Buddhist art, it is more related to Orthodox iconography than Buddhism proper. The empty seat of traditional Indian Buddhst art implied that anyone could sit in that seat i.e. each of us has potential for buddhahood.
In contrast, Kushan Buddhist art was influenced by Greek sculpture, and subtly stated that few people would achieve buddhahood.
IMHO most Buddhist statues of Bamyan were not of Buddhas but were mostly of bodhisattvas.
As well, statues of the Buddha represent not the historical Buddha but his deity-form and deeper concept of Buddhism in the form of an absolute Buddha i.e. they represent deeper spiritual representations than the historical Buddha.
Thus, Buddhist sculpture evolved to represent deep spiritual concepts that go beyond Gautama Buddha and his biography.
Hence, in creating these works of art, their nameless creators were in effect demonstrating non-self in action.
