The KK principle doesn't imply that everything that is true is already known. That's a very different claim. There are people who endorse it--Kant does, for instance. But many contemporary philosophers would take this to lead to some kind of pernicious idealism.
What the KK principle says is that everything you know, you know that you know. If I know paris is the capital of france, then I know that I know that paris is the capital of france. In other words, knowledge is transparent. It is impossible for me to have some knowledge which I am unaware of.
The reason to hold some thesis like this is a claim about the nature of knowledge. If you think to have knowledge is to have some evidence, or something like that, then you'd be attracted to the principle.
On the other hand, if you think that knowledge can be produced by reliable belief forming processes or something that operate whether you are conscious of them or not, then you'd want to deny the principle.
it is clear that you can fashion consistent epistemic logics either with or without the KK principle, so the question whether the KK principle is true or not isn't a logical, but a philosophical question.