Pretty straightforward answer; there just wasn’t a lot of sweeping changes to Tier 6 content after it was released. We did a pretty thorough investigation of the data of each patch between 2.1 and 2.4.3, as well as looked at public patch notes and just came to the conclusion that there wasn’t enough of substance for an interesting pre-nerf difficulty.
Most “changes” were essentially bug fixes, with only Mother Shahraz and Archimonde seeing anything resembling a real mechanical change, and even they were actually fairly minor. Contrast that with Tier 4 and Tier 5 that had some major (and highly visible/memorable) mechanical changes happen between patch 2.0 and 2.1 and it just made a lot of sense to really dig in and go big with pre and post-nerf for those tiers.
Sunwell Plateau will have a pre and post-nerf state however, as there were more significant changes there.