We know Bhishma Pitamaha was a legendary warrior. He always fought for the right thing and for truth.
Then, if Kamsa was doing so much evil, and Bhishma Pitamaha was already there on Earth, why didn't he try to stop Kamsa?
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.
When Kans wanted to kill devki, Vasudev(father of Lord Krishna) came in between and said don't kill devki as she has not done anything wrong and their son won't kill his own mama, however kans didn't agreed to which.
After that vasudev said that he and devki would surrender to kans and also promised that none of their relatives(including hastinapur where bhisma pitama was senapati) would attack kans to free vasudev and kunti and in return let devki live.
In Hindu mythology, Kunti (Sanskrit: कुंती Kuntī) also called Pritha, was the biological daughter of Shurasena, a Yadava, the sister of Vasudeva, the foster daughter of her cousin King Kunti-Bhoja, the wife of King Pandu of Hastinapur and the mother of King Karna of Anga and King Yudhisthira of Indraprastha.
Source : kunti
Kans agreed to that and then he kept devki and vasudev in prison.
The other reason why bhishma didn't stop kans was he was bound to the throne of hastinapur and he couldn't attack any one without kings consent and since kans didn't do any harm to hastinapur, he and bhishma pitamaha never came face to face.
Remember, Kamsa was son-in-law of Jarasandha and who inturn was very very powerful. Jarasanndha was supported by many asuras. Just imagine all these asuras coming together (Jarasandha, Kamsa, Narakasura etc) and fighting the Kurus..Bhisma et al would have been laid down flat in no time, however strong they might have been.
Krishna and Balarama eliminated all these asuras directly or indirectly before Kurushetra war. Otherwise all the asuras would have sided Duryodhana in all probability and Pandavas couldn't have dreamt of a victory. Then Lord and his elder brother himself would have been forced to take up their weapons to destroy all these and fight them all simultaneouly in Kurushetra.
Also, we know, Lord Krishna or Balarama didn't kill Jarasandha too, even though Jarasandha was at the mercy of Balarama 17 times, he was pardoned by Krishna and Balarama. Jarasandha was killed by Bheema only because of Lord Krishna's help. Otherwise, even Bhima cannot kill him.
So, firstly it is not easy to kill Kamsa in proper war, as all asuras might have come together and no Bheesma would have been sufficient to face and kill them all. May be the entire Kuru race would have destroyed, even before pandavas or Kauravas were born had bhisma waged a proper war against Kamsa and other powerful asuras.
Secondly, it was destined that Kamsa should be killed by Lord Krishna. Only, Krishna could kill Kamsa and not any one. This was one of prime reasons for Krishna avatara.