It’s probably to protect the stairs from the elements as they can get slippery when wet and I’m assuming metro line doesn’t want any incidents to occur
This space is fascinating because it didn’t exist in any way, shape or form before the revitalization even began. Heck the York concourse even existed as the car rental area beforehand
Another detail I noticed when passing through Union was how from platform level where the stairs to the bay concourse are I could see a floor sticker. This could further hint to its opening being near