Good question! I think there are two main reasons here.

First is legality. Psilocybin truffles are legal in the Netherlands, so retreat facilitators can openly run their psychedelic retreat without fear of imprisonment.

Second is the perceived sacredness of the plant. While LSD most certainly has effects similar to psilocybin, it does not have the same rich and long-running history as other psychedelics. For example, Ayahuasca has been used for hundreds of years in the Amazonian basin in spiritual and religious ceremonies. So, facilitators in the Netherlands may believe that this sacredness is important for their ceremonies.