In Mark 6:5, we learn that Jesus ‘could not’ work miracles in Nazareth, with the exception of a few healings. Why couldn't He do more healings here? V.6a reveals: “He marveled because of their unbelief.”
To be blunt: It was the Messiah's general practice to heal people of faith rather than dole out His miracles to skeptics and adversaries. John Haley comments: "Mark 6:5 implies not physical but moral impossibility. It was not lack of power which prevented His working miracles at Nazareth; but as the next verse shows, the unbelief of the people was the reason why He 'could not' thus work.”
A conscientious father tells his wild teenage son: "I cannot give you the car keys. Not until you get your behavior under control." Such a father could physically give car keys to his son, but is morally unable to do so. He cannot morally do such a thing.
When Christians approach ‘Good Friday’ of Holy Week, we recall something else the Messiah could not do. When Jesus was being crucified, his enemies actually spoke some truth as they sarcastically declared: "He saved others; Himself He cannot save" (Matt.27:42a). The Messiah could not come down from the Cross and thus be our Savior. If He saved Himself from the cursed death on the Cross, human sinners would remain cursed and perpetually unsaved. Messiah Jesus experienced the wrath and curse of God for our sin (Gal.3:13); He died on that Cross so that His people of faith could be forgiven and reconciled to God.
John Calvin expressed it this way: "The Son of God determined to remain nailed to the cross for the sake of our salvation, until he had endured most cruel torments of the flesh, and dreadful anguish of soul, and even death itself. Christ, though he might easily have done it, did not deliver himself from death...And why did he for the time disregard his own safety, but because he cared more about the salvation of us all?"