Let me begin by correcting those who like to associate poverty as some state of mind and not actual living condition. The standard definition of poor refers to having little or no money, or other means of supporting ones living. This simply means that if a person lacks the financial strength to support their livelihood to a point of being comfortable and having at least an extra little to spare on savings, then such a person is poor.
Going back to the question, I don't think all hardworking people are poor. Millionaires were not made by fluke, they actually worked hard to where they are today. But I guess your question is referring to those majority of labor workers who fall below average living conditions. If so, then it's not hard to point the reasons why.
Modern day society has made it a must to have some sort of educational training and certificates to show for before securing any reasonable job that might eventually put you on or above average living conditions. The majority of those hard working folks drawing from your question's narration are those who lack acceptable working education, thereby leading them to work menial jobs, especially labor intensive jobs. As we we know, those types of jobs pay very little and anybody who relies on such jobs for earning a living will be classified as poor.
Destiny is the inevitable or necessary fate to which a particular person or thing is destined or a predetermined course of events considered beyond human power or control. Though some people do not believe in destiny, I like to think that destiny plays a very vital role in our lives. Believe it or not, somethings are just meant to be as they tend to be beyond our controls, porverty included. Take for instance a poor couple living somewhere in the slumps of a remote third world country giving birth to a physical challenged child. Odds are going to be highly against such child from succeeding in life. This is because as long as their living condition remains the same, there's really nothing such a family can do to bring forth a positive change. That's what I refer to as destiny. And a lot of different situations are largely influenced by factors associated with destiny because they often are beyond our control or powers to correct.
So yes, I think some people are destined to remain they way there are, be it poor or otherwise.