It is impossible to single out one reason for the fact that the Dutch have more cycle paths than any other nation in the world. Many factors contributed to this phenomenon. But is interesting to look at one of the historic aspects.
This video is not a comprehensive explanation of the development of transportation in the Netherlands. This video is just highlighting one of the many aspects that made the Dutch think a main road is not complete without a cycle path. The video focuses on the ‘how’ it happened not on the ‘why’ it happened.
That is why it also doesn’t mean that a history like this is a condition to build good cycle infrastructure. Examples of good practice in the Dutch situation (adjusted for local conditions) could be copied anywhere.