No one seems to really know, but most people think it's because on the tree grapefruits hang in clusters like grapes. When grapefruits are small (green) they especially look like a cluster of unripe grapes. The grapefruit is a hybrid, a cross between a pummelo and an orange.

It's because you're pronouncing it wrong.

It's g-rape fruit. When fed to nigs it causes uncontrollable rape rage until they satisfy their needs with rapery.

Without grapefruits black people would be kangz. It's so sad. We should cancel grapefruits.