The world has yet to churn out another mind quite like Einstein's, that flips convention on its head, or opens up humankind's collective knowledge by an order of magnitude. Special people, like Einstein, this man, Norman Borlaug or Ada Lovelace, I think, may have just been the right people at the right moment.

Having read somewhat widely, as a kid, about the history of technology, I was struck by how often several people were working independently on the same ideas at the same time, but that is not so surprising once you realize that the timing is often determined by when some prerequisite knowledge became available, or some prerequisite capability was mastered. Einstein stands among the most original thinkers of all time, but even he was standing on the shoulders of people like Heaviside, Fitzgerald and Lorentz (and Maxwell, of course) - yet the first three, at least, are invisible in popular historiography.

It is a case of having the right person in the right place at the right time, Einstein would have not had his theories had he been around 20 years earlier, or if he had not had the opportunities that he did. And if he had not been around then I think it would have been others. It is about the environment, Calculus was developed by two people simultaneously.

As to why we haven't had someone who has done this, well who's to say that we haven't and they are just overlooked? Or that greater capital expenditure it has to be groups of people (finding DNA, cloning, Graphene, etc). Or that there has simply not been the right environment.

Draw a line between these and choosing what field of study or what kind of change to the world counts to make a "special person" changes the number of special people dramatically.