a combination of many things.
For one, Cities in the netherlands are already quite dense, and the dutch are focused on building family houses attachted to each other mostly (row housing).
Also, thanks to the massive agricultural sector and a lack of oversight on industry, the netherlands has a massive problem with nitrogen in its soil which prevents building because building stuff generates more nitrogen.
Speculation and the liberalisation of the housing market has also massively contributed to price increases.
https://en.wikipedia.org/wiki/Nitrogen_crisis_in_the_Netherl...