I have a lot of family over there. Based on what they've said and what I've observed, it's due largely to the following reasons.

1. It's super flat.
2. Driving a car is a pain.
2a. Gas is expensive.
2b. Driving-related taxes are expensive.
2c. The roads are tiny.
2d. There's very little parking.
3. The country is tiny, so the distances between locations are typically much smaller.
4. The bike infrastructure is awesome (both a cause and effect of the large cycling population).
5. The average Dutchman is in better physical condition than the average American.
6. Although probably a more minor reason than the aforementioned, the Dutch I encountered were more environmentally concerned than most people I know in the United States.
7. Culture* (See below)

*In my limited visits to the Netherlands and Germany, I got the impression that driving was viewed as much more of a luxury than it is in the United States. For example, I'm 21 and I don't know even one person my age who does not have a driver's license. By the time I was a senior in high school, I didn't know even one person in my class who did not drive. Most of them even owned a car.

In the Netherlands (and also Germany) though, nobody I met in the age range of 16-20 years old owned a car or even had a driver's license. In the Netherlands this was primarily due to the short distances and the prevalence of cycling (as well as a combination of factors listed above), and in Germany (Munich) this was due to the excellence of the public transit system (as well as some of the same reasons listed above for the Netherlands). If the topic of driving or vehicle ownership came up, locals my age were amused that I owned a car, and they acted like this placed me in some elite, privileged category. I eventually tried not to bring it up because their reactions made me feel so pretentious.