As per the promise given to Pandavas and Kauravas about usage and distribution of his resources, he wasn't supposed to participate in the Mahabharata war, rather he was only supposed to be an adviser.
Krsna with his power could have rescued Abhmanyu easily, but his promise prevented him to do so..
Sri Krishna was lying down, resting, when Duryodhana and Arjuna arrived. Duryodhana entered first, and sat near the head of Sri Krishna. He grabbed a chair and reclined. Arjuna did not sit on a chair. He stood with folded hands at the feet of Sri Krishna. After a while, Sri Krishna opened his eyes, and first, naturally, he could see only what was near his feet, not his head.
“Arjuna, how come you are here at this moment?” said Sri Krishna.
“No sir! I have come first,” said Duryodhana from behind.
“Oh! You have also come,” said Krishna.
Sri Krishna said, “You have come first, but I saw Arjuna first. Also, he is younger, you are the elder. Don’t you think it is proper for me to speak to the younger one first, especially as I saw him first?”
Then turning to Arjuna, Sri Krishna asked, “What made you come here?”
Arjuna replied, “Great Master, you know what is going to happen. War has become inevitable. We all want your help.”
Sri Krishna said, “What can I give you? I have two things. I have a large army called Narayani Sena; if you want it, you can take it. Otherwise I am here, but unarmed, doing nothing. I will merely sit and discuss with you. I will not take part in the war. If you want such a man as I am, take me. Or if you think this is not going to be of any utility to you, take the large army which will help you, as it is almost invincible.”
“I want you only, Master,” replied Arjuna.
Immediately Duryodhana retorted, “I want the army.”
“Take it,” said Sri Krishna.
Duryodhana left the place hurriedly, and declared to the Kuru family that he had already won victory in the war, that his victory was certain because of the invincible forces that he had received from Sri Krishna.
When Duryodhana left the place, Sri Krishna accosted Arjuna and said, “What a foolish person you are! Why did you not ask for the army? What good is it if I sit idle without doing anything for you? Why have you made this wrong choice? The other man took the good forces, and you are asking for me, who is as good as nothing.”
Arjuna replied, “Thou art all for me, great Master. I know you very well. Don’t try to deceive me by this query as to why I have chosen you.”
“Oh! You want to vie with me. Okay, all right. Do that,” said Krishna.
Then they both left.