Why do historians consider 1942 to be a pivotal year for the outcome of the war?

There are a lot of reasons for this! 1942 features many intense moments for the world, including the Battle of the Coral Sea, the Battle of Midway, Stalingrad, and the Manhattan Project. I think the most important is the Manhattan Project, because this is what is usually publicized (at least in American history) as the "final straw" of the Allies.