As can be seen
here we discussed this in the rules forum.
Main reason to do this is the counter-intuitive interactions with these cards.
For example, a 2/2 creature enchanted with armor of thorns (resulting in a 4/4) that receives 2 damage would die under the old rules (first, the end of turn trigger of the armor of thorns would destroy it, BEFORE the clean-up step). With the new rules, the sacrifice takes place during the clean-up step, so the critter does survive.
Another change it with necromancy. From now on, it is not possible to abuse the end of turn trigger trick. Under old rules, you could resurrect a creature after the end of turn trigger, so you would have the creature also in your next turn. With the new situation, the creature will only survive one turn.
Main reason to do it NOW is the introduction of Mirage on Magic Online.