Really, like most big issues, there is no one, simple answer.
Some are poor because they are too lazy to do anything about it.
Some are poor because they have mental problems.
Some are poor because they were born poor and never had the resources to get out of being poor.
Some are poor because they are genuinely just down on their luck(got fired and regardless of best effort, couldn't find a new job, something like that).
Some are poor because they can't get over their addictions(alcohol, gambling, crack, whatever).
Of course, there's also this conception that the poor are all miserable. This isn't true. Many are happy, or at the very least content, with their lives - sure they might like a little extra money, but money doesn't buy happiness(if it did there'd be no miserable rich people). Some are indeed miserable. Some are happy in some ways and miserable in others.
Ultimately though, there will always be poor people - as long as there are those that are better off than others, the people with the least will always be considered 'poor'. Even if you elevated everyone's standard of living to the point where the poor live like rich people do now, and rich people live even more luxuriously, the poor will still be considered the poor and the rich the rich.
Great breakdown John D.