Because for those of us who want to use nested quotes, it’s a hell of a lot easier to have the system not auto-strip out the inner quotes than to go laboriously go searching back up through the thread to re-quote each inner post (and inner inner post and so on) into the appropriate place.
The board worked this way for years and no one ever complained. People are only complaining about the switch back now because they’re not used to it. Soon enough, they’ll be used to it again.