CapnZapp
Legend
Because the devs made the assumption that HD was a poor gauge on the monster's "level". They likely thought of 3E upsized monsters like beasts and zombies that had loads of hit dice, yet presented a relatively modest challenge. IIRC you could have twenty or thirty hit dice and still only be an appropriate challenge for a level 10 party...why, WotC, why not HD...?
Maybe Dave2008 has concluded this... caution... wasn't necessary in practice, but that only means the difference is slight enough to not matter.