Any question answered...

Why do quill stems not have open faces (in general) like ahead stems?

At the time, many AHead stems had closed bar clamps too. I think the move to detachable clamps was mainly a matter of manufacturing simplicity once people started making forged stems with tubular extensions, but the introduction of tight bends on road bars made sure they stayed that way.

Is there some article that tells the story of when quill stems transitioned?

Not sure about one article, since it was a swift but far from instantaneous transition. DiaCompe patented the AHeadset in 1990. A few other things were happening at the same time which made people pay attention.

1. Yet another mountain bike boom, but this time they were serious because
   
2. Suspension forks were about to get workable
   
3. The cold war ended and defence cuts left a lot of people with experience of high quality small run manufacture looking for an outlet
   
   

So, suspension fork makers could cut inventory due to needing only one steerer length, and a lot of boutique stem builders could swap very easily to the new design. It probably didn't hurt that steerers grew to 1¼" and then shrunk back to 1⅛", so people didn't just automatically keep using the old headsets they'd grown up with.