Unlike first-class monads, sum types are not some sort of esoteric FP construct that arises from challenging technical problems around dealing with external state. They are the very obvious dual of product types, which every language has some version of. They are easy to understand, require very little tutorial-ing, and their power is immediately obvious when you start using them.

Product types model "x and y", sum types model "x or y". One of them is universal. The other has been totally missing from mainstream languages until recently. No wonder our software sucks so much.