I think this is more of a version of the "why have we stopped making good music/movies, almost everything today is garbage?" issue. With the benefit of hindsight, we only remember the greats (and the very worst villains) from past eras. While if we look around today, we see all the people who will never make it into the history books, just like we see all the songs and movies that will never become "classics." Edit:…
Second thing is that it was a lot easier to be a genius in 1900 than it is now.
Not saying that general relativity is well understood by general public but a lot more people has now some grasp on E=mc^2. While in 1900s it was something that most of people could not wrap their heads around.
Last point is that "geniuses" are overrated anyway. Because what we need as a species is that bell curve of knowledge moves up. So mediocre people get more intelligent and know more things and people from lower part of bell curve get to the level where mediocre people were before.
We achieved that because currently average Joe nowadays is much smarter than average Joe 100 years ago.
And we should strive to move forward with that.