It is because god specifically told them not to eat it.
Your great great great great great grandfather ate a fruit 6000 years ago and now we're all going to hell, BUT god sacrificed an innocent man who committed no sin (and was also God) to suffer our punishment instead of us and that means nobody has to go to hell but only if you have faith that he will save you AND accept the innocent man that was killed as your lord and savior and if you do then you also get to spend all day and night worshiping god for all eternity.
