The obvious answer is that we don't know, and that's all the answers anyone can give without going into speculation.
That said, I'd rather expect it to be a temporary omission because of the big FaceTime redesign in iOS 12. Apple probably wants to nail the group FaceTime feature along with its new interface and functionality first and foremost, and they might just not have found the time to squeeze in that feature properly in time for the iOS 12 release, similarly to how many other bigger app redesigns have temporarily removed features that then came back a year or so later. I believe there's even still an on/off switch in the iOS 12 beta for Live Photos during FaceTime, which would support this theory. The feature isn't even that old; there isn't a feasible reason to deliberately remove it permanently.