From what I had heard from a friend, Halo 2's Legendary Difficulty was a Bungie's answer to too many people complaining that Halo: CE's Legendary Difficulty was not hard enough. In Halo 2, they made it to where enemies do a ridiculous amount of damage, can take a lot of damage, and are hyper aware to your location when you are within eyesight of them.
Apparently, too many people then complained that Halo 2's Legendary Difficulty was way too hard, so Bungie scaled it back some when working with Halo 3's Legendary. As someone who just started a solo Legendary run in Halo 2 after just completing a solo Legendary run in Halo 3 (yea, for some reason, I started with 5 and worked backwards through the titles), you are basically made of glass covered in wet tissue paper in Halo 2. So much as 4-5 individual rounds from a Plasma Rifle are enough to kill you. And, unless you are lucky, melee is instant death if you happen to get too close or are bum rushed by the enemy.
It's not, really, but because of the rushed development. Originally, each encounter was supposed to have more enemies, however Bungie feared that having so many enemies in a single area would be more detrimental than good, and there's good reasons for that:
- Too many enemies shooting at a single target, the player;
- Each fight becoming shooting galeries like Halo 5;
- The framerate would tank with so many AI present in the map.
To counter this, they reduced the numbers to what we see in the final product, but they had to buff the entities and, well, they overbuffed them a bit and had barely any time to playtest, resulting in the unfairly balanced difficulty we currently know.