The creators of the large-scale sculptures found in cave temples were clearly limited in choice of medium -- whatever type of stone was  found at a chosen site. The statues of smaller scale, however, could be made from a variety of materials. Cost was a significant factor. Bronze was a more expensive material and technically more difficult to manipulate, and so was often reserved for small statues like the one you just saw. Unfortunately, no material is absolutely permanent. Bronze images are easier to preserve since, in comparison to stone and clay, they do not break easily, but over the centuries there have been periods of economic and social upheaval which have caused the melting down of bronzed objects.