Albert Einstein was a genius, Kurt Gödel was a genius, John Von Neumann was a genius (I hope that people over here will share the meaning of "genius" which I understand, for me I always find Von Neumann being described as "genius" that is very highly intelligent and very much above an average species' mind), Alan Turing was considered a genius, Wolfgang Pauli was considered a genius etc.
We hardly find Rene Descartes, Galilee Galileo, even Isaac Newton and James Clerk Maxwell being described as geniuses, they are simply termed as good theorists. Philosophers are almost never described as geniuses, Bertrand Russell's (an example of 20th century) philosophies are not celebrated much (the answer that he was proven wrong by Gödel is almost same as saying Catholics were proven wrong by Protestants and hence doesn't tell us anything).
There is a difference between the way Aristotle is revered and the way Einstein is revered, what has caused that have been making us to refer post 19th century scientists/mathematicians as geniuses? Why there number has increased almost like a "step-function" after 19th century?
Pierre De Laplace, Karl Frederich Gauss, and few others of 18th-19th centuries were prodigies (similar to today's "genius") but then also they weren't called as very much above the human species' mind, all they did was that they learned things quite early in their age.