What is the wisdom behind not answering prayers against an oppressor, even though Almighty God "is capable of restoring my rights from him"?

The wisdom of Allah Almighty in His divine decree (qada') and destiny (qadar) cannot be encompassed by human knowledge, and the believer must submit and yield to it. This world is not a realm of recompense or a scale of absolute justice; absolute justice will be in the Hereafter, as Allah Almighty says: "And We place the scales of justice for the Day of Resurrection, so no soul will be treated unjustly at all." (Quran 21:47). Allah may grant respite to the oppressor in this world as a means of gradual enticement (istidraj); if justice is not taken from him in this life, it will be taken from him on the Day of Resurrection. The oppressed will be granted victory by Allah, either swiftly in this world or later in the Hereafter. As for supplication (du'a), Allah will not let its effect be lost on His believing servant, and the forms of its answer vary between granting what was asked, averting a similar evil, or storing it for the Hereafter.