Twelve hours after casting this question into the ether and so far the only two answers focus on a very specific type of prosperity: financial. That's not surprising; yet it seems an extremely narrow perspective for a word that has huge potential. There are as many ways to prosper as there are people to define the boundaries.
In addition, it seems unreasonable to ask God why hard-working people fail to prosper. Try to find a god who doesn't both define prosperity and explain in great detail what it takes to achieve it. No... God - any god - wants people to prosper and tells them how, so it's unnecessary to ask.
Just the same, we all know good people who work hard and follow the rules but somehow miss the mark when it comes to filling the prosperity basket. More than likely, it boils down to lack of attention.
They could have good health, but they don't pay attention to the information readily available that will help them find it and keep it.
They could have good relationships, but they don't pay attention when they see the same problems cropping up repeatedly in their lives. Or maybe they recognize the patterns but don't watch for sources that could provide good solutions.
They could have peace and harmony in their lives, but they don't notice how they allow certain people to take that away from them. Or maybe they see what people are doing, but don't find ways to make their influence impotent.
They could have a financial cushion to fall back on or to use to their advantage, but they don't notice all the little habits that daily eat away at their resources. Or maybe they notice the things that fritter their money away, but complain while denying the potential for change.
There are many ways to prosper that have nothing to do with money. A person who lives in fear due to the influence of evil people in his life might happily trade his money for a carefree life lived on a tight budget. Someone with terminal cancer or debilitating pain could easily be imagined trading financial prosperity for a healthy life. Someone who recognizes the value of their beloved family, trusted friends, well-honed skills and the beauty of their world might never miss the kind of prosperity suggested by both the posed question or the initial answers.