Besides some of the obvious reasons such as the previously mentioned location limitations and low education limiting possibilities, I'll throw out another one.

Besides some of the obvious reasons such as the previously mentioned location limitations and low education limiting possibilities, I'll throw out another one. Once you're in a job like this it kind of limits your ability to branch out into less physically demanding and better paying work. Your resume kind of pigeon holes you with respect to future employment and the fact that you're working long, exhausting days means its very hard to find the time and energy to build your skills outside of work.