That has been the bad choice making of the poor, bad decision making.
Without any doubt our good decisions regard wealth are the ones that make us financially stable or you can say rich, and if we aren't able to take good financial decisions then we'll either be middle class or poor when it comes to finances.
But, it's not just our decisions but our habits as well that can impact the level of our wealth, a few weeks ago I read a story of a gambler who lost more than $100M in gambling if I remember it correctly, and that loss was due to habit of gambling which we all know could cause addiction.
If someone who works hard and maintains good balance of spending and investing in some good assets and who's emotionally strong enough to hold that investment for some years then that person won't remain poor for long time, but surely hard work plays its role when you're very poor, however once you make some money then it's on your mindset that either you'll grow it or spend it.