Because at some point in the recent past, Sylvanas made a deal with the Jailer, and from that point, all souls were being funneled into the Maw; this has "broken" the cycle of death, because what is supposed to happen is when you die, the Arbiter places you where your soul needs to be, amongst presumably others, the 5 zones we will see in SL. I imagine we'll find out why we are the lucky ones to go and fix this mess in the pre-patch
As I was typing this, I realized that the main folks who wrote this storyline are fans of the TV show "The Good Place".