Actually, this is a very subjective question. Who knows what someone can achieve and not achieve and what is more or less than this.
Assume someone has achieved motherhood and a desk job. If they are happy with this achievement, is this all they CAN achieve or is this all they want to achieve? Is this more or less, according to who? Who chooses what each person can achieve, then? Their mother? Their peers? Or themselves?
Quite hard to tell, I say. I guess many people have the ability to change the whole world, but I don't think the whole world wants to be changed.

Part lazy, Part comfort zone, Even a storm follows the path of least resistance.

It can be what applebomb said or that they are truly content with less or settle for less for their own personal reasons. No one really knows exactly why except for that person themselves.

Because if you set the bar low enough, it's impossible to fail.

It can be a personality disorder, such as Borderline Personality...one of the symptoms of it is that the person doesn't live up to their potential....

It could also be a parent in their home growing up, role modeled "less than" behavior...

Because they are lazy, or lack drive and confidence. Or, maybe they are just happy how they are and happy with what they have.

Just to manage less stress in life Just what is your life worth if you have to constantly prove yourself every day against conventional thought? Achievement can take on a life of its own. That's why John Lennon went to being a private individual, he had no balance of private vs public life.When is enough, ENOUGH?

For fear of failing. It's hard to see yourself in the right light all the time. For instance, I have always thought I have a wide frame for a girl. My friends happens to be a stick, right? Tall and thin. No curves, but she doesn't need them. Somehow or another, even with my curves, I am smaller than her.

There are areas of everyones life that are underestemated my the liver.

Low self esteem. People won't shoot for the stars if they don't think they deserve it. And fear. People are afraid of having they're dreams crushed, especially when it's something that means so much to them.

Because of what they have to sacrifice in order to get there.

aim low and u would never have to face dissapointment

Because it's easy or they're comfortable with where they are.... and people don't often even realize what they are capable or achieving.

Because it's enough for them.