Finn is the son of Shara and Kes Bey, whom literally planted a "Force Tree" in their goddamn backyard at the end of issue 4.
What's interesting is that both Finn's parents were Rebels., yet Finn himself is pitched as an ex-Stormtrooper. This might indicate that Imperial Operation: Cinder (render key worlds uninhabitable ) mighthave actually been successful; thus stunting any attempt to forge a New Republic, and allow the Empire the breathing room it needed to quash the Rebellion.
This would adequately explain why everyone is scattered and relegated to myth by the open of TFA.