Why does Allah make life difficult for some of His slaves and severely restrict them, to the point that they become incapable of doing anything?

It is astonishing that someone should resent the decrees of Allah, while Allah is showering him with His blessings. This is indeed ingratitude (kunud), which is to count misfortunes while forgetting the blessings of Allah upon him. Allah is on a straight path, and all His legislative and cosmic decrees are justice, wisdom, and benefit. Allah is more merciful to His servant than a mother to her child, and He decrees nothing for the believer except that it is good for him. The benefit might lie in the difficulty of affairs, either as purification from sins, or to elevate ranks, or to hear supplications. So, let the servant submit to the command of Allah. Allah's giving and withholding are a trial and a test. Not everyone to whom sustenance is expanded has been honored by Him, nor has everyone whose sustenance is restricted been humiliated by Him. Honor lies in knowing Allah, loving Him, and obeying Him, while humiliation lies in turning away from Him and disobeying Him.