[Philip-J-Fry]

It's the same reason you shouldn't wave pedestrians across any other multi lane traffic, because you can't always see what's coming in the other lanes. Some turnings, especially on wide roads, put the car and the pedestrians waiting to cross quite far away.

Pedestrians only had right of way if they're in the process of crossing. The new rule is that pedestrians waiting to cross have priority, which is just not practical to introduce without everyone being on the same page.

Being predictable is how you are the safest pedestrian or driver. Prior to the rule change being predictable was assuming pedestrians wait for vehicles. With the rule change, you can no longer be predictable because different people have different expections of what a pedestrian or driver will do.