Do you think Kamsa was some nobody that Bhishma could just invade and kill him ? Kamsa was extremely powerful and had defeated Indra and the Devas in battle.
- Description of Kamsa’s strength from Harivamsa -
O lord of the world, having slain the army of Jarasandha and vanquished the other enemies in battle why hast you met with death at the hands of an ordinary man? Alas! Fighting with Indra in a combat of arrows you were not defeated by the immortals. How hast you been then slain by a mortal? Having agitated, with a shower of arrows, the ocean that is incapable of being moved you didst conquer Varuna the holder of the noose, and take away all his riches. When Vasava did not pour profuse showers, you didst, for the citizens, bore through the clouds with your arrows and bring down rain by force. By your prowess all kings were humiliated and used to send you the precious jewels and clothes.
- Chapter 31, Vishnu parva, Harivamsa.
2. Vishnu Purana says that Kamsa defeated the Devas including Indra -
I scorn the Ādityas, accompanied by the timid Vasus. I scorn the Agnis and all the other deities defeated by my mighty arms. Did I not see Indra, king of gods, retreating from the battlefield with arrows in his back? He couldn’t take them on the chest. When Indra stopped the rains from falling on my kingdom, did not the clouds, split asunder by my missiles, release their showers, just as I desired?
- Chapter 4, Book 5, Vishnu Purana.
3. Srimad Bhagavatam also says that Kamsa defeated the Devas -
What can the gods do with all their endeavours. They are cowards on the battlefield. They live in perpetual terror of the twanging of your bow-string. Being beaten on all sides with volleys of arrows discharged by you, they, being anxious to save their lives, deserted the battlefield and took to their heels. Some gods in their wretched plight laid down their arms and stood with folded palms, while others with their loincloth untucked and hair on the head untied and dishevelled announced, “We are afraid.”
- Chapter 4, 10th Skandha, Srimad Bhagavatam.
4. Garga Samhita has an elaborate description of Kamsa’s battle with Indra -
With a hard punch Kaṃsa struck the great elephant and with a second punch he struck Indra in that battleground. With that punch Indra fell far away and the elephant was wounded and fell, its knees touching the ground. Again standing, the king of elephants attacked the demon-king Kaṃsa with its tusks, picked him up with its trunk, and threw him eight-hundred thousand miles. His body hard as a thunderbolt, when he fell he was only a little upset in his heart. Angry, and his lips trembling, he returned to the battleground. Kaṃsa grabbed the elephant-king Airāvata, threw him onto the battleground, strangled its trunk, and broke its tusks into pieces. Knocking over many great heroes, the elephant Airāvata fled the battleground and went to the capitol of the demigods. Taking the bow of Lord Viṣṇu and stringing it, with a flood of arrows accompanied by the twanging sound of the bow, the demon-king Kaṃsa made the demigods flee.
- Chapter 7, Goloka Khanda, Garga Samhita.
So Kamsa was an extremely powerful ruler. He was also aided by powerful Asuras. Bhishma probably don't want to risk a battle with Such a powerful king. Only Lord Krishna could have killed Kamsa easily.
