Why earthquakes generally take place at plate boundaries?

The plates are either moving past each other (transform boundary), toward each other (convergent boundary) or away from each other (divergent boundary). In some cases when there is a convergent boundary, one plate will be subducted under the other plate. During this process and the process occurring at a transform boundary, the plates don't slide easily past each other because they are jagged. The plates can get locked. Stress continues to build. Eventually, the stress becomes so great that the plates "pop" free. When that happens, there is a tremendous amount of energy released in the form of an earthquake.