Here's your answer:
1) The worlds a bigger place then it was 30 years ago - in fact it has added about a billion more people. More people means more outliers
2) There are a lot more Africans in general running and and Kenyans and Ethiopians in specific, and they are running longer and with better training and resources. Many happen to be very good runners, this increases the pool of world class athletes - for every 1 Henry Rono, now you have 15 and many of the best ones hang around longer.
3) US population has undergone the same boom in pop that begat those great running days. This has led to a lot better times.
4) Even without drugs, science and technology have helped elite runners recover faster and, since they can earn a living doing so, run fewer races but at a higher level.
5) Great running means more great running - elites feed on the performances of other. Look how many broke 4 within 5 years of Bannister.