There are several reasons why so many people are poor in the United States. These are not "statistical research" reasons. I live in a very poor neighborhood, and these are the things I see.
Dashingscorpio has some great reasons.
1. The area is bad. Years ago, my area had a bunch of steel mills that provided jobs for people. Then the steel mills closed down, and nothing took their place. The town is rife with vacant buildings and empty lots. The only jobs available within 25 miles are minimum-wage retail jobs, and a handful of small factories that pay between $7.95 - $10/hour. Working full-time at one of the plants will net you a monthly income of $1272 - $1600 a month before taxes. Expenses for food, utilities, housing, transportation, etc. easily cost that much and more. An easy solution would be to get up and move to a better area, but that costs money that people just don't have.
2. Addiction to drugs or alcohol. Eats up your money and just makes it impossible to do constructive things with your life.
3. Overspending habits.
4. Too much debt. Lots of people go to college and get student loans, then find themselves in a job where they're not earning a livable wage. For example, even though they have $20,000 debt, they're earning $8-$10/hour and earning a living which is under the federal poverty level. Add a car loan on top of that so you can get to work, and you're buried and stuck.
5. Having a college education does not ensure a decent job. I spent $30,000 on a college degree and was never once able to find a decent job with it. I work in the factories like everyone else.
6. Kids. Kids are expensive, and the more you have, the harder it is to get your head above water. Pets, too.
7. Something bad happens. You get sick and get stuck with huge medical bills, you get laid off, divorce, etc.
8. A very large percentage of my neighborhood population has a history of arrests and incarcerations. They are unable to get the low-paying jobs that do exist because every company around here does a background check and just chooses not to call you if you have a record, especially if it lists a felony or two. So mostly they just keep doing crime to earn a living. Their options are pretty limited.