Many dungeon bosses in WoW are designed in such a way that they get progressively more difficult the longer they are dragged out for and become unmanageable after a certain time. I'll call this "time scaling". Perfect example is last boss in Court of Stars.
So when dungeon boss encounters become 40% longer because of Tyrannical (which is about 3+ key levels of hp), it suddenly becomes much harder to manage the encounter because it is almost certain that "time scaling" will kick in. In addition, many more mechanics scale to 1 shot territory very quickly.
Compare this Fortified where most mobs do not have "time scaling". Most mobs do the same thing until they die (or in a lot of cases, nothing really). That makes it so you can mostly get by Fortified weeks with enough attrition even if output is low.
This creates an imbalance between these two affixes. Output requirements are much tighter in Tyrannical weeks and as a result, the Tyrannical affix is generally much harder to handle, particularly in the non-cutting edge level where players do not have the luxury of cherry picking group compositions.
Possible solutions to this problem are:
- stop designing boss encounters with "time scaling", or;
- add more "time scaling" trash mobs and slightly reduce the +hp of both tyrannical and fortified. I think this would balance out participation between the Fortified and Tyrannical weeks.