Ah, HistoryFan and his questions ! 
Why are there so many very talented driver from the Netherlands now. And why not in the past?
Depends on how you define "past" and "talented", for one thing. In addition to those mentioned in the original post, there have been many other Dutch drivers who have achieved (some degree of) success in international motorsports in recent years, or are still doing so, for example:
Christijan Albers, Phil Bastiaans, Jeroen Bleekemolen, Yelmer Buurman, Nick Catsburg, Tom Coronel, Carlo van Dam, Robert Doornbos, Stef Dusseldorp, Cor Euser, Christiaan Frankenhout, Mike Hezemans, Patrick Huisman, Francesco Pastorelli, Nicky Pastorelli, Peter Kox, Jaap van Lagen, Xavier Maassen, Nigel Melker, Jos Menten, Bas Schothorst, Pieter Schothorst, Ho-Pin Tung, Jos Verstappen, Renger van der Zande ...
But talent aside, without either a sponsor/personal benefactor or a spot in a junior programme (e.g. Verstappen/Red Bull, De Vries/McLaren) it is virtually impossible to reach Formula 1, therefore many talented drivers end up in other categories. That is a persistent problem for Dutch drivers, especially as Dutch companies tend to sponsor teams rather than individuals.
In general though, I reckon there is nothing special about a sudden wave of talent, or talent dry spells for that matter.