It’s the “pull up the ladder behind you” mentality.
Sad to say, this applies to a lot of aspects in life, it’s not unique to WoW really.
Not enough people play this game anymore that you have a constant stream of new players for low keys, etc.
So those areas just dry up, intentional or not.
People just finish and move on, everyone else is kinda boned, even if the content is still ‘avaliable’, and they come back for the next patch, and rinse and repeat.