The nearest planetary body in the sky to us in tide-locked, so it must be pretty common, right? Tidal locking is not random.
Well the idea of becoming tide-locked lies within the physical mechanics of planetary bodies. All space-bodies revolving in space will become tide-locked in a matter of time (if it is only heavily affected by one graviational source. It's only a matter of time. (and the gravitational force between the bodies, rotational inertia, etc.) (Note: magnetic attractions could also possibly cause tidal-locking. Just substitude "magnetism" for "gravity" in the subsequent sentences.)
Basically, this phenomenon occurs due to the center of gravity (CG) being off of of the axis of rotation. This causes the force of gravitational attraction to be stronger on one "side" of the planetary body (the side the CG is on) than the other side. This torque causes an eccentricity or "wobble" in the revolution/rotation. The body undergoing tidal-locking will slowly experience slowing down rotation/speeding up revolution or speeding up rotation/slowing down revolution. Eventually, the period of rotation and revolution will equal each other. In addition, the larger body will eventually tidally-lock itself around the smaller one, as well. (i.e. Pluto and Charon)
The systems known to be tidally-locked within our solar systems are listed by Wikipedia as:
The Moon (to Earth); Phobos and Deimos (to Mars); 8 moons (to Jupiter); 15 moons (to Saturn); 4 moons (to Uranus); 2 moons (to Neptune); and Charon and Pluto in twinned tidal-locking, in addition to numerous other bodies predicted to be tidally-locked.
The only planetary bodies that will not be tidally-locked are those that undergo disruption of this process, perfectly-spherical bodies with perfectly distributed mass (very, very, very unlikely) or those that enter orbital resonance. (Such as Mercury)
Well why have no planets in our solar system undergone this? This is probably because scientists believe that our sun is relatively small (in comparison to non-dwarfs), and that our solar system is relatively young. However, this does not mean such phenomena will not occur in other planets.
I apologize for the perhaps overly-lengthy reply, but I hope this satisfies any questions!
