Everyone here saying lore reasons has already been proven wrong by the devs themselves.

4.0 will not require the completion of HW to enter, you will be given a brief summary of HW and you are on your way to new content.


This exact thing could have been done for ARR<HW. This was an unintentional oversight by devs, not something they did for lore reasons and the fact that they have adressed this and stated it will not be the case anymore for future expansions only proves this