Lore wise, I believe it is because we have slowed the Burning Legion from their ever-increasing invasion, and now we are going to where the largest pocket of them snuck through before we pushed them back. Though in a true, living, war, they would still get through randomly to areas like Deadwind Pass and Winterspring no matter how hard we reinforced the Crossroads or Sentinel Hill.
Edit: adding more
I think that as a gameplay mechanic, it kind of deters us from playing our mains in the Broken Shore. So, as a gameplay mechanic, it makes sense to remove it to force us into the expansion zones. Though lore wise and content wise, they should honestly make them random for each zone - kind of like the Deathwing event where he killed a bunch of people randomly.
It would make more sense to allow it into every zone in Azeroth except maybe Vash'jir. This would just turn it into a fun way to not exclusively quest or afk in SW/ORG for dungeon ques. It would make sense for this route to work exclusively as that zone's last level (i.e. Elwynn was like 9 or 10) for the mob levels and you have to be in range to get into the event. But then comes the problem of it takes too long...
I honestly would like it to turn into a smaller system for every zone closer to Rift's Rift events.