I'll tell you why. Because of perspective.
Take for instance, the Hungarian doctor, Semmelweiss who worked out that newborns were far more likely to die if their mothers had been delivered by doctors who'd been attending post-mortems immediately before working in the labour wards. Babies delivered by midwives tended to survive. Because nobody understood germs, his fellow clinicians rejected the idea that doctors should wash their hands before attending a birth. In fact they were offended by it. Semmelweiss became increasingly distraught because he wasn't believed, and eventually died in an asylum.
Semmelweiss published his theory in 1847. From our distance, we can see so many extraordinary relevances to this story. Not least that it concerns a Hungarian, and Hungarian voices are being stifled as I write. But because from a distance Semmelweiss was so obviously astute, and wise, and at the time people could not see that because they were locked into their own way of doing things. We need distance, and we need to reflect on the past, and dwell in the past, so that we can understand who we are, and where we have come from, and why we don't want to walk that way again.