Perhaps it's done to ensure a certain level of mathematical maturity. For example, here is what one author writes in the preface to his discrete mathematics text:

This book has been written for a sophomore-level course in Discrete Mathematics. [. . .] Students are assumed to have completed a semester of college-level calculus. This assumption is primarily about the level of the mathematical maturity of the readers. The material in a calculus course will not often be used in the text.

(Eric Gossett, Discrete Mathematics with Proof, 2nd ed., John Wiley and Sons, 2009)