Imagine trying to harness the power of lightning to convert it into usable electricity. To make this task on par with tapping into an earthquake, add to that the complication that you never know when a storm is going to show. Furthermore, once you have your equipment in place, you can never move it. As impossible as that challenge appears, that's more or less what energy companies would be dealing with if they ever considered earthquakes as a power source.
Unlike storms, predicting an earthquake to within a matter of hours, days or weeks simply isn't possible with what seismologists understand today about faults, despite these scientists' best efforts. At best, seismologists can predict the likelihood of a quake along a fault line taking place within a matter of a few decades. Given that consumers don't want power next year but rather right away and at all times, an earthquake can be a pretty unreliable power source.