“Rather odd that this phenomenon had a greater impact on a subset of Jews than other parts of humankind.”
I was thinking along the same lines — so that perhaps a corollary of this thesis would be to ask why the Arabs ceased to evolve about 800 years ago...
A more likely explanation is that the type of intelligence that Ashkenazi Jews developed due to evolutionary pressures during medieval and early modern periods just happens to be a type that fits in very well with success in the modern world.
Other types of intelligence may be more effective at aiding survival and reproduction in other environments.
For instance, stubborn independence would be a highly disadvangageous trait for slaves, however admirable by today’s standards. Challenge the master too many times and win a Darwin Award.
Or they could be devolving. That is, they’re getting dumber, not smarter. The dark side of the study asserting Jews are smarter is that other groups of humans are dumber. No one wants to go there for political reasons, even if the science beckons and it’s true.