Because we also use Pyret to teach program complexity, and big-O is a whole 'nother can of worms in Haskell (the cost model is totally non-traditional). Because we use Pyret to introduce students to state, and even basic equality is a whole 'nother can of worms in Haskell. Because we teach basic algorithms in Pyret, and even something like DFS in Haskell is complicated (because of the preceding issues).