Edit: What follows reflects the original title, "Why is functional programming not taught in schools," Schools have teachers, not professors. Schoolteachers do not write their own textbooks.
The textbook companies from which the teachers are allowed to purchase their materials are the largest problem. The textbook companies are quick to jump on "the next big thing", which was OOP a few years ago. Functional programming has fallen by the wayside. Many teachers cannot or are not allowed to teach a course without a textbook, so the course selection generally follows the availability of textbooks from the big vendors.
