I think with knights, people at least feel like they have a chance, because most lists bring some sort of heavy support. Maybe you will get some lucky rolls, he gets some unlucky saves, and you still manage to blow up a knight. And then you can play objectives with the rest of your army. Maybe you will still lose in the end, but its a maybe.
But say you are facing some 200 model horde army. Turns take forever, plus all your fancy toys you brought are doing nothing because they are killing cheap models (and there's another 199 more on the table). And he is winning the objective game to boot.
Ask me if I rather face an all knight army, or an all horde army, and I know what my answer will be.
Knights have some obvious weakness. They are super low model count, so its really hard for them to play objectives.
IG on the other hand, was already good before the codex, and after the codex, they were super good. I struggle to see what weakness
IG has. Their stuff are cheap in points, they can field hordes, they have
CP coming out of their ears, they can play the objective game easily, and they can field super shooty stuff too, some of which don't even need line of sight, and their strategems, traits and such boosts all their strengths to further levels. They are mobile too (have you seen an infantry squad move under the orders of move move move? the distance is breathtaking).
So what's their weakness? Their fragility? Is a horde of infantry really that much more fragile than say a squad of terminators? A knight is gonna have a lot more trouble killing a bunch of 40 conscripts compared to a squad of terminators. And Bullgryn aren't fragile by any means either.