Was there a geographical reason for the US to implement the Truman Doctrine?
The Truman Doctrine applied to all countries everywhere, so in that sense there was no geographical reason for it. However, there was a geographical reason to announce it with regard to Turkey and Greece, which were the two countries Truman discussed in his speech announcing the policy.
In terms of strategy, Turkey and Greece are important because they control the approaches to the Black Sea. The Black Sea is where the Soviet Union had its only warm water sea ports. For this reason, both the USSR and Russia before it had wanted control of the Dardanelles and the Bosphorus which control access to the Black Sea. The US did not want the Soviets to control this region because that would have given them unlimited access to the Mediterranean for their warships.