The search results did not directly provide the specific reason for the demolition of Willem van Eijk's farm in Harkstede. Based on what is commonly known, Willem van Eijk, known as "Het Beest van Harkstede" (The Beast of Harkstede), was a convicted Dutch serial killer. Farms and properties associated with notorious criminals are sometimes demolished to serve various purposes, such as eliminating a site of public curiosity, avoiding it becoming a morbid tourist attraction, and giving closure to the community and victims' families.

For detailed insights or the official reasoning behind the demolition, further investigation or more specific local sources might be necessary. The Wikipedia page on Willem van Eijk or documentaries like "Het Beest van Harkstede" may offer additional context or leads on more detailed local reports.