It is indeed somewhat surprizing why it took so long to figure out grapefruit juice (and it was by accident during the study of the effects of coffee on a drug). Some of the early drugs that interacted with grapefruit juice where short-term agents like antibiotics, and others like channel blockers didnt have obvious detrimental toxicity though they might have had effects that were above the range of typical human variability. Powerful statins became household items a bit later, and all the cancer drugs were invented after this association was already known.