Question
Why didn’t Bhishma Pitamaha stop Kamsa?
Answer
There's no reason to believe that Bhishma would try to stop Kamsa. You can ask the same question about a million different people: why didn't Bhishma kill Kamsa, Shishupala, Jarasandha, Narakasura, etc. And why didn't Drupada kill any of them, and why didn't the Pandavas kill any of them (other than Bhima killing Jarasandha)? It's not the dharma of a Kshatriya to go around the world killing all the evildoers. The dharma of a Kshatriyas involves enforcing Dharma within your own kingdom, and possibly going outside your kingdom to enforce Dharma if war is justified by Vedic law in a particular circumstance.
Jarasandha was a bad guy for a long time, and he even drove Krishna and the Yadavas out of Mathura, but the Pandavas never tried to fight him during any of that. It was only at the time of Yudishtra's Rajasuya Yagna that it was justified for Bhima to go to Magadha and challenge Jarasandha to a battle.
Note: “The question: Why didn’t Bhishma Pitamaha stop Kamsa?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.