That's an interesting point. I guess it's clear that the runners of 40 years ago were disadvantages compared to today's runners (equipment, training, diet etc.) so the fact they were so close is pretty impressive!
If you look at other events though (say the mile), the times have come down a lot more.
My theory is that things like equipment, training and diet are (relatively) less important for the 100m. By which I mean that the top sprinters are 'born' that way - OK so it still depends a lot on training etc but perhaps not so much as say middle distance running.