I could be totally wrong, but my hunch is that this is more related to the lowest hanging fruit rather than human capacity. Maybe scientific discoveries simply get exponentially harder. In painting, it's very rare for someone to make anything original these days. We won't have any more Picassos, etc, simply because they were the first to explore a new terrain. It's really not a matter of education, genetics, or talen…
I agree the "lowest hanging fruit" is likely a key factor, as well as tuttoring. But I would also guess that being recognized as a genius was easier in past times, especially for aristocrat family members. The same way music artists will never be as huge as the Beatles or Michael Jackson once were because of the long tail effect, we now are exposed to so many bright minds, and not way to pick out an Einstein amongst…
Another point is that geniuses might not be recognized as such since we still haven't seen the long terms impacts of their work. Eg: Van Gogh.