Why is preschool education popularized from high classes to low classes instead of from low classes to high classes?

Let me give you an idea, which is also my own understanding. Children can go to kindergarten at the age of three, then to primary school, and then to preschool in a few years. So from here, preschool classes are high classes (high classes) and kindergartens are low classes (low classes). Popularization from high classes to low classes is because compulsory education is popularized from primary schools, and preschool classes are most closely related to the teaching content of primary schools, so high classes will be popularized first. For example, many people after 90 may not have attended kindergarten, but they must have attended preschool. Now kindergartens are gradually popularized, and almost all of them will go to kindergartens after 90. This is probably the popularization from the high class to the low class.