Remember. A cannibal human is still a Humanoid (Human) for all mechanical purposes. Were we to change its type, items and game mechanics specifically affecting humans would cease to work and that's not especially desired here.
D&D 3.5 defines the Monstrous creature type as "Monstrous humanoids are similar to humanoids, but with monstrous or animalistic features." Being a cannibal is a behavior. While that may certainly be seen as barbaric and monstrous, it's not in itself a creature type feature.