A: Better understanding of the game and their abilities
B: +Healing Gear that didn’t exist at this point in the game originally. +spellpower as well. These things simply did not exist in 1.0, so your damage and healing were much less, which made people have to reduce the number of engaged mobs to make it do-able, hence the need for CC. When your level 40 healer has +200 healing, it makes things pretty easy.