From comments by user2227713 on this answer:
Why hasn't Paizo done more to balance these low-tier classes? Don't get me wrong, new classes are great, but I'd like to see balance changes too.
Rather than answering that in another comment, I'm turning the question over to the community: What reasons has Paizo had for not changing the core classes' tiers?
The Pathfinder CRB provides 11 classes: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, and Wizard. When played vanilla, these classes are T4, T3, T1, T1, T4, T6, T4, T4, T5, T2, and T1, respectively. When played with all Pathfinder published material, the tier of each class is roughly the same, maybe allowing a one-tier change.
It does seem deliberate, since additional material affects the power dynamics between classes so little, and since whenever additional material does have the potential to alter balance, it gets errata in such a manner as to ensure that doesn't happen. If it were entirely due to neglecting balance considerations entirely, one would expect additional material to correct or reverse the errors by way of random chance, if nothing else. What reasons do the Pathfinder developers have for maintaining the original power discrepancies between classes?
Answers from developer quotations are preferred, but experience working for or with Paizo might also work. As usual, remember to "Back it Up", since speculation is not an acceptable answer to SE questions.
Best Answer
You know how the first step in solving a problem is admitting you have a problem? Paizo does not acknowledge any balance problems on the extreme levels described by the tiers. Everything else stems from that.
Jason Bulmahn, lead designer of Pathfinder, describes core as the balance point for the game:
This is, of course, a reasonable position on its face. He is certainly correct that using later books to improve matters is problematic for the disparities that can cause (though of course, it still happens to some degree). But the problem is that this assumes that core itself is at least a reasonable, stable base to maintain. The tier system argues this is untrue: if you read the descriptions of the tiers, you will see that the gaps between characters—particularly spellcasters versus martials—is extreme. And this answer kind of pretends that the state of core is not their responsibility—see the offloading of responsibility onto 3.5—which of course is not true, they wrote the Core Rulebook and have the authority to errata it. If the tiers are accurate, it would certainly seem that it very much is appropriate to do so, since after all no class is billed as struggling to do anything or capable of mastering everything, but Bulmahn dismisses the possibility.
And that is because they deny the tiers themselves. While they may acknowledge that things are imperfect, they consider the problem mostly minor, denying the extreme problems posited by the tiers. And they are not shy about this opinion—they have gone on the record insulting and denigrating those who claim the problems are severe and in desperate need of addressing. For example, James Jacobs, creative director of Pathfinder, on the subject:
That means that they just keep repeating the same problems over and over. They write new combat feats, but they're balanced against the old ones, actively prevented from getting better—and any better feats that sneak through consistently do get errata-nerfed. They write new spells, but they don’t nerf the old ones, so the new spells are either just as high-power, or are ignored for considering the power of the class with access.
And ultimately, there are just fundamental differences between spellcasting classes and non-spellcasting classes. Spellcasting classes get way, way more spells than non-spellcasting classes get features. That enables massively more versatility than is even possible on non-spellcasting classes. Without actual changes to the classes themselves, that will never get fixed.