Why is Finn the last human in Ooo?

Answer 1

Because, after the Mushroom War, almost the entire humanity has fallen because the effects of the nuclear bombs, the survivors went to the underground, to protect themselves from mutagenic diseases and radioactive dust, but when they were underground, they did not know that above their heads the world was changing.

As you can see, Susan Strong was a Survivor human, that went to the underground to protect herself and the life of her fellows from radiation. But, after a time, the radioactive dust and mutagenic diseases affected them. This is just a theory, however, it´s a good theory, because in AT you can see many evidences of this war (Ocean of Fear: the cars buried underground and destroyed buildings; Memory of a Memory: in the background of Marceline's house, you can see a tank, a burning city and ruined buildings; Beautopia: a ruined city built underground obviously to protect humans from the mutated creatures and a chance to renew humanity, and many others).

Maybe Finn survived because of luck, he went out of his house or maybe he was in the surface and survived the war luckily, while his parents died in his place to protect him. I think these things will be clarified soon in the next AT's episodes. I´m glad if i clarified your question.