Season 2 achievement points
It’s just the way Season 2 Achievements work. Fortunately for those that were not enthralled with this mechanic, later Seasons don’t require using the same character.
At the time they were promoting the fact that Season 2 was permanent and infinitely re-playable, unlike Season 1. So the idea was the first time through you focus on the story and then repeat it for achievements (and unlike Season 1 they were actually a challenge, not simply grinding kills/clicks).
I don’t know why they didn’t update it later on when the novelty of repeating episodes later wore off and having to do it on every character just to attempt the achievements on them proved unpopular, but that was the reason originally.
“Life’s a journey, not a destination.”