European kings were busy consolidating their power. Few of them hoped for benefits for themselves from participating in the campaign, and harm could be if, in the absence of the king, neighbors or vassals who did not go on a campaign attack his lands. At this point, the pope could not give the kings a guarantee against such attempts; later the authority of Rome grew and it became possible.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.