I am not sure if your question is why most/many hexapods have legs with three degrees of freedom, or if you are asking about this particular look.
If it is the look, I think a lot of people possibly starting with the MicroMagic stuff and later with Xan with his phoenix and the like made it look like this because of the look. Also these legs also have a good range of motion.
Mechanically some of the other leg designs like some of these from Lynxmotion:
have a mechanical advantage of being able to support more weight as the brackets supports both ends of the servos and does not put all of the stress at the servo horns.