Is not lowering one's gaze from the forbidden a cause for lack of blessing in sustenance, and why are many of those who let their gazes roam and commit sins still provided for?

Allah expands and restricts provision as a test, not as an honor or a humiliation. What matters are righteous deeds. Wealth is not a sign of honor; it may be a trial. Poverty is not a sign of humiliation; it may be an endurance. Sins have negative consequences: they remove blessings, bring calamities, and restrict provision. As for a man looking at a non-mahram woman, it is forbidden due to its dire consequences for the heart, and it contravenes Allah's command to lower the gaze. The husband must repent from sins so as not to expose his family to harm.