Chapter fourteen opens with Krishna making a remarkable claim. He tells Arjuna: I will share with you, once more, the knowledge that stands above all other knowledge. Every sage who understood this teaching attained the highest state of perfection.
Think of an elder in the family who has many stories, but keeps returning to one particular lesson because it matters more than anything else. Krishna is doing the same here. He has spoken about many things across the previous chapters, but the knowledge of the three gunas is so central that he returns to it with fresh emphasis.
The word "bhuyah" (once again) is significant. This is not new information dropped without warning. Krishna has touched on the gunas before. Now he is going to lay it all out in detail, because understanding sattva, rajas, and tamas is the key to understanding why we are the way we are.