I do not think your assumption is correct. Let me clarify why -
Ravana - he was not so powerful. kArtavIrya Arjuna manhandled Ravana very badly, he crushed Ravana in his powerful arms like how Garuda holds a serpent in his claws and there was nothing Ravana could do (You can find it in uttara kAnda of Valmiki Ramayana or read it here). vAli was way more powerful than Ravana, i.e. a vanara was more powerful than a descendant of Brahma. You can read that here. So how exactly is Ravana 'extremely powerfull'?
Duryodhana - he was defeated easily by Arjuna single handedly, you can read it in virata parva of Vyasa Mahabharata. He was also defeated by Bhima twice in Kurukshetra war per Vyasa Mahabharata much before the Drona was even killed. Read it here, which is a good summary of the war. He had to be constantly protected by Bhishma, Drona, Karna, Ashwadhama in the Kurukshetra war. Not 'extremely powerful' was he!
Kamsa - how did you get the impression that he was extremely powerful? He always relied and depended on hired help (per Vyasa Bhagavatam). At the end a boy ended up killing him. Not 'extremely powerful' is he!
Evil is as strong as good is, sometimes it is more stronger than the good and vice versa. And balance to equilibrium is reached quickly if that imbalance occurs, depending on which yuga it is. That is the norm. If that wasn't the case, then bhagawAn would not tell Arjuna in Vyasa MB BG that "..para dharmO bhyavaha ha" But evil is certainly not much more stronger than good is.