sml is a smaller language, and a "cleaner" one in some ways. if your goal is to teach students the elements of functional programming, you want them to be able to concentrate on the core concepts, and not on the incidental details of the language.

http://adam.chlipala.net/mlcomp/ is a very good overview of the differences between sml and ocaml, which lets you see some of the tradeoffs each language has made.