Acting White has an interesting post up, Do Black People Avoid Costco? He thinks that black folks just need to learn to stretch their time horizons and learn to budget better.
I can't speak for black people, but I can speak for the entrenched poor. Poverty, by definition, is not having enough resources to live on. It's not a matter of better budgeting. Lemme give you an example.
A poor person smokes cigarettes. They cost (depending on where you live) $8 a pack. Poor person wants to quit smoking. Smoking cessation aids (nicotine gum, patches, medication) all cost money, slightly more money than cigarettes but over the long run the savings will be substantial. To get started you need to lay out more money than you would to just buy a pack of smokes to get you through right now. You can manage to scrounge up the $8 on a fairly regular basis, but saving up the $60 to get started on the gum is impossible. If you could just quit smoking for the week or so it would take you to save your cigarette money for smoking cessation aids, then you wouldn't need the smoking cessation aids. So you keep buying smokes, a pack at a time.
Now for Costco, you need to save up the $60 for the membership fee. If you could save that much buying groceries at a regular store, then you wouldn't need to spend the $60 on a membership fee. (And we won't even get into the costs of transportation with giant bulk items of food. Think you can do your monthly Costco run on a city bus with a kid or two in tow. Ha!) It is, like everything else in the world, much more expensive to be poor because you don't have the little bits of money that grant you the ability to make life easier or cheaper. It's the difference between payday loans and credit card interest, check cashing fees and bank fees, etc, etc.
I do have a theory as to why this seems to be less of an issue with "working-class Latinos, Asians, Whites, and everybody else", and that would be the hopelessness factor. There is a difference between new poverty or short-term poverty (working class whites, who until recent generations fared all right), the poverty of new or recent generations of immigrants, and the generational, perpetual, entrenched poverty. The first two kinds still have some kind of hope. They haven't seen, over and over, the sacrificing of the bottom 20 percent to the lions. They could get out of this with the right job, or if they educate their kids right. At least it's better than where they came from. There's a million things they can tell themselves that make the idea of saving for a rainy day a hopeful act. But when you've seen generation after generation go through all the things they are supposed to do to better themselves only to be shot down over and over and over again, you realize that you can't save for a rainy day if it never stops raining. If you come from that kind of desperation, the hard work and sacrifice required to perform middle class home economics isn't rational. It will all be for naught. Better to hunker down, do your best, and find what little bits of joy you can, even if it's in the form of a deadly cancer stick that makes pretty smoke rings.
I would bet that communities that have been here longer (Native Americans, African Americans) have higher levels of hopelessness than newer immigrant communities. Not to erase the Latin@ Americans and Asian Americans who have been in this country for centuries, but the longer a group has been at the bottom of the resource pyramid, the less benefit they are going to see from acting middle class.