A large factor in how a material breaks has to do with how quickly the material can break/form bonds. Glass is known to be extremely slow to respond at a molecular level (in fact, it's somewhat of the definition of "glass"). This means that as you deform a glass, it is unable to change it's structure in time to heal the damage done and accommodate the new shape you're trying to put it in. This leads to the material failing. In metals, and other ductile materials, the atoms can break/form bonds much easier, which leads to the material being able to accommodate the strain you put on it when you bend it. Also, materials have different ways of spreading out the strain you put on them along them which lets them bend slightly and uniformly instead of sharply and in one place like glass.