The question should be closed, for these reasons:
First, the title is not the real question. The title is:
Why do the tiers of classes change so little regardless of what additional material is used
and the questions bolded in the content are:
What reasons has Paizo had for not changing the core classes' tiers?
What reasons do the Pathfinder developers have for maintaining the original power discrepancies between classes?
Considering the tags the title indicates that the question is about how each additional Pathfinder material is changing the tier list. A phenomenon is identified ("additional materials in Pathfinder doesn't make the tier-list change"), it's on-topic because related to RPGs.
Then by reading the core of the question it shifts to: "I know the developers are trying to do that, what is their motivation?". I agree a small change in meaning is not always avoidable but here it could be easily corrected by editing the title to something like "What is the motivation of Paizo behind preserving the tier-list?" which would be less misleading.
Secondly the whole question is imputing motives without at all considering that maybe its premise could be wrong. I presented an answer with tangible elements that contradict it and the reaction of the OP is "I'm pretty sure we can all agree is false", without presenting any element in this direction.
Finally, I think these bolded questions are simply not very nice. Maybe not un-nice enough for requiring anything, but it adds up with its other issues.