People focus on willingness/ability to clean when discussing this topic, but individuals also vary in their affinity for creating filth in the first place. I think the latter is what really makes the difference.
I have a brother who lives in filth, while I am reasonably clean. I don't really devote much time to actively housecleaning, but I just don't really make messes in the first place.
For example, if I want a glass of orange juice, I pour the juice, put the OJ container back in the fridge, drink my juice, and then put the glass in the dishwasher. If I happen to spill a few drops at some point during the process, I take the 5 seconds necessary to wipe it up with a dishcloth right then and there. Three minutes after pouring my juice, no sign remains that I even had a glass of OJ.
If my brother has a glass of OJ, on the other hand, he leaves both the used glass and half-empty OJ container wherever they happened to be sitting when he was done with them. If he spilled a bit, he just leaves it there. A week later, his now moldy OJ glass is still sitting somewhere in his bedroom and there is still a sticky spot on the counter where he spilled while filling said glass.
Multiply the above by the hundreds of individual small messes one could create in a day, and my brother is living in filth while I am living in a clean home.
As far as how this relates to poor people, I guess more detail oriented people are likely to not be big mess makers while more scattered people are likely to leave messes. This might have some imperfect correlation with earning potential