。
Because no one would believe this story.
Literal
Anyone this story [object-を] believe-not [probably-だろう] [because-からだ].
誰も...ない 'no one' is built from the interrogative 誰 + も + a negative verb, creating universal negation. だろう expresses conjecture 'probably, would be.' ~からだ 'it's because...' is a sentence-ender explaining the reason for something previously stated — a fixed construction for '(it's) because.' The sentence stands alone as a justification clause, typically following an unstated claim ('X happened — because nobody would believe it').