How can one explain the dominance of ideas about the immutability of species in the 18th century?

With the establishment of the dominance of the Christian church in Europe, an official point of view based on biblical texts spreads: all life is created by God and remains unchanged. Such a direction in the development of biology of the Middle Ages is called creationism. A characteristic feature of this period is a description of existing species of plants and animals, attempts to classify them, which for the most part were purely formal (in alphabetical order) or applied (in the nature of human use). Many systems of classifications of animals and plants were created in which individual characters were arbitrarily taken as the basis.

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.