Why didn't you answer what makes him almost impossible to understand?

*not

What's there to answer? He writes unclearly.

If I may add—there's a new translation of Critique of Pure Reason by Yirmiyahu Yovel that is more or less readable and understandable. At the beginning there's an introduction and summary of Kant's doctrine that is practically a book in itself (almost 100 pages), and it's fairly clear.
If you want something lighter (relatively speaking), there's the Open University course book called Studies in Kant's Introductions; the material there is written very nicely.

But I really would be glad to hear a response to the original poster's question—why are so many masterpiece books written in an unclear, cumbersome, hard-to-understand way? It seems to me that you also mentioned Popper's book as being hard to get through. Is it just that they happened not to know how to write clearly, or is there something about systematic writing at a deep level that comes together with this kind of awkwardness?

For some of them, it was an ideology of concealment and directing the text to intellectuals. That's the esoteric approach. Others try to be precise, and so they get drawn into hairsplitting and specialized terminology in order to avoid mistakes and inaccuracies. And some simply lacked writing ability or didn't invest enough time in clarity (as Mark Twain wrote: sorry for the length, I didn't have time to make it shorter).